This is one Goliath of a thread, so I've categorised it by gamemodes. Click on the "spoiler" tags below to view a specific game's proposed achievements, or click on them all if you want to view the thread in its entirety.
Total: 99 achievements
May add more later on down the line, but considering how time-consuming this was, I wouldn't count on it.
Title: 100 achievements
Requirements: Suggest 100 new achievements on the forums
Reward: Owner rank
Skywars
Title: Triple tap
Requirements: Kill three players in rapid succession with a bow
Reward: 150 experience, 50 points
Title: Light the torch
Requirements: Imbue a beacon with the 'Strength' buff
Reward: 150 experience
Title: Icarus
Requirements: Fall from the top of the world to your death
Reward: 50 experience, 20 points
Title: Iron defence
Requirements: Protect against ten arrows with a shield
Reward: 50 experience
Title: Heaven ascent
Requirements: Complete all Skywars achievements (Excludes 1,000 & 10,000 wins)
Reward: 5,000 experience, 500 points 5 cubelets
Title: Triple tap
Requirements: Kill three players in rapid succession with a bow
Reward: 150 experience, 50 points
Title: Light the torch
Requirements: Imbue a beacon with the 'Strength' buff
Reward: 150 experience
Title: Icarus
Requirements: Fall from the top of the world to your death
Reward: 50 experience, 20 points
Title: Iron defence
Requirements: Protect against ten arrows with a shield
Reward: 50 experience
Title: Heaven ascent
Requirements: Complete all Skywars achievements (Excludes 1,000 & 10,000 wins)
Reward: 5,000 experience, 500 points 5 cubelets
Survival Games
Title: Slow and steady
Requirements: Kill six players with an axe in one game
Reward: 250 experience, 50 points, 1 cubelet
Title: No hunger in this game
Requirements: Acquire a total of thirty-two food items in one game
Reward: 150 experience, 30 points
Title: Vulcan
Requirements: Fall into the volcano
Reward: 30 experience, 10 points
Title: Pluto
Requirements: Kill a player whilst wearing full gold armour
Reward: 100 experience
Title: Canine companion
Requirements: Feed two players to the hounds in one game
Reward: 150 experience, 1 cubelet
Title: The mother lode
Requirements: Craft a piece of diamond armour
Reward: 250 experience, 1 cubelet
Title: There's more where that came from
Requirements: Kill one player
Reward: 10 experience
Title: I'm just getting started
Requirements: Kill ten players
Reward: 15 experience, 5 points
Title: Ruthless
Requirements: Kill one-hundred players
Reward: 100 experience, 10 points
Title: Relentless
Requirements: Kill one-thousand players
Reward: 500 experience, 250 points
Title: Final Boss
Requirements: Kill ten-thousand players
Reward: 5,000 experience, 2,500 points, 5 cubelets
Title: What's yours is mine
Requirements: Steal somebody's chestplate via fishing rod and equip it
Reward: 100 experience
Title: Bandit
Requirements: Steal and plunder somebody's Smuggler chest
Reward: 120 experience, 30 points
Title: War glutton
Requirements: Complete all Survival Games achievements (Excludes 10,000 & 1,000 wins)
Reward: 5,000 experience, 500 points, 5 cubelets
Title: Slow and steady
Requirements: Kill six players with an axe in one game
Reward: 250 experience, 50 points, 1 cubelet
Title: No hunger in this game
Requirements: Acquire a total of thirty-two food items in one game
Reward: 150 experience, 30 points
Title: Vulcan
Requirements: Fall into the volcano
Reward: 30 experience, 10 points
Title: Pluto
Requirements: Kill a player whilst wearing full gold armour
Reward: 100 experience
Title: Canine companion
Requirements: Feed two players to the hounds in one game
Reward: 150 experience, 1 cubelet
Title: The mother lode
Requirements: Craft a piece of diamond armour
Reward: 250 experience, 1 cubelet
Title: There's more where that came from
Requirements: Kill one player
Reward: 10 experience
Title: I'm just getting started
Requirements: Kill ten players
Reward: 15 experience, 5 points
Title: Ruthless
Requirements: Kill one-hundred players
Reward: 100 experience, 10 points
Title: Relentless
Requirements: Kill one-thousand players
Reward: 500 experience, 250 points
Title: Final Boss
Requirements: Kill ten-thousand players
Reward: 5,000 experience, 2,500 points, 5 cubelets
Title: What's yours is mine
Requirements: Steal somebody's chestplate via fishing rod and equip it
Reward: 100 experience
Title: Bandit
Requirements: Steal and plunder somebody's Smuggler chest
Reward: 120 experience, 30 points
Title: War glutton
Requirements: Complete all Survival Games achievements (Excludes 10,000 & 1,000 wins)
Reward: 5,000 experience, 500 points, 5 cubelets
Blockwars
Title: Olympian
Requirements: Capture 1,000 flags
Reward: 500 experience, 250 points, 3 cubelets
Title: Not the objective
Requirements: Kill fifteen players in one game and still lose
Reward: 250 experience
Title: Murder hole
Requirements: Kill four players in rapid succession from within your castle
Reward: 300 experience
Title: Castle crasher
Requirements: Capture a flag in the first ninety seconds of the game
Reward: 200 experience, 1 cubelet
Title: Tactical evasion
Requirements: Steal the enemy flag whilst cloaked
Reward: 50 experience
Title: Tactical retreat
Requirements: Capture the enemy flag with only one heart left
Reward: 100 experience, 1 cubelet
Title: Olympian
Requirements: Capture 1,000 flags
Reward: 500 experience, 250 points, 3 cubelets
Title: Not the objective
Requirements: Kill fifteen players in one game and still lose
Reward: 250 experience
Title: Murder hole
Requirements: Kill four players in rapid succession from within your castle
Reward: 300 experience
Title: Castle crasher
Requirements: Capture a flag in the first ninety seconds of the game
Reward: 200 experience, 1 cubelet
Title: Tactical evasion
Requirements: Steal the enemy flag whilst cloaked
Reward: 50 experience
Title: Tactical retreat
Requirements: Capture the enemy flag with only one heart left
Reward: 100 experience, 1 cubelet
Eggwars
Title: Shell of envy
Requirements: Equip full diamond armour
Reward: 50 experience
Title: Filthy feathered
Requirements: Kill an enemy that you debuffed with weakness
Reward: 50 experience
Title: Raising the steaks
Requirements: Purchase a stack of steak
Reward: 100 experience, heart problems
Title: Cannibal
Requirements: Consume chicken (chicken to be dropped when a player is burnt to death)
Reward: 5 experience, 1 point
Title: Armour piercing
Requirements: Destroy an egg with an arrow shot from a bow (eggs to be breakable via bow & arrow)
Reward: 150 experience, 20 points
Title: Plucked
Requirements: Kill a total of 500 players in Eggwars
Reward: 500 experience, 100 points
Title: Chicken massacre
Requirements: Kill a total of 1,000 players in Eggwars
Reward: 1,000 experience, 250 points, 2 cubelets
Title: Shattered beaks
Requirements: Kill a total of 10,000 players in Eggwars
Reward: 5,000 experience, 1,000 points, 5 cubelets
Title: Eye of the Eyrie
Requirements: Complete all Eggwars achievements of all categories (Excludes all win achievements)
Reward: 10,000 experience, 1,000 points, 10 cubelets
Title: Shell of envy
Requirements: Equip full diamond armour
Reward: 50 experience
Title: Filthy feathered
Requirements: Kill an enemy that you debuffed with weakness
Reward: 50 experience
Title: Raising the steaks
Requirements: Purchase a stack of steak
Reward: 100 experience, heart problems
Title: Cannibal
Requirements: Consume chicken (chicken to be dropped when a player is burnt to death)
Reward: 5 experience, 1 point
Title: Armour piercing
Requirements: Destroy an egg with an arrow shot from a bow (eggs to be breakable via bow & arrow)
Reward: 150 experience, 20 points
Title: Plucked
Requirements: Kill a total of 500 players in Eggwars
Reward: 500 experience, 100 points
Title: Chicken massacre
Requirements: Kill a total of 1,000 players in Eggwars
Reward: 1,000 experience, 250 points, 2 cubelets
Title: Shattered beaks
Requirements: Kill a total of 10,000 players in Eggwars
Reward: 5,000 experience, 1,000 points, 5 cubelets
Title: Eye of the Eyrie
Requirements: Complete all Eggwars achievements of all categories (Excludes all win achievements)
Reward: 10,000 experience, 1,000 points, 10 cubelets
Duels
Title: GG
Requirements: Win a duel with less than one heart left
Reward: 125 experience
Title: Taste Vengeance!
Requirements: Rematch a lost duel and win
Reward: 50 experience, 20 points
Title: Big Bang
Requirements: Win an Explosive Bow duel via void
Reward: 100 experience
Title: Brass knuckles
Requirements: Win a 1.9 duel by punching your opponent to death
Reward: 100 experience, 50 points
Title: Madman
Requirements: Win three duels in a row
Reward: 100 experience
Title: On the lamb
Requirements: Win five duels in a row
Reward: 150 experience
Title: Serial killer
Requirements: Win nine duels in a row
Reward: 300 experience, 50 points
Title: Terminator
Requirements: Win fifteen duels in a row
Reward: 500 experience, 2 cubelets
Title: Liquidator
Requirements: Win twenty-five duels in a row
Reward: 750 experience, 100 points, 3 cubelets
Title: Town takeover
Requirements: Complete all Duels achievements (Exclusives 1,000 & 10,000 wins)
Reward: 5,000 experience, 500 points, 5 cubelets
Title: GG
Requirements: Win a duel with less than one heart left
Reward: 125 experience
Title: Taste Vengeance!
Requirements: Rematch a lost duel and win
Reward: 50 experience, 20 points
Title: Big Bang
Requirements: Win an Explosive Bow duel via void
Reward: 100 experience
Title: Brass knuckles
Requirements: Win a 1.9 duel by punching your opponent to death
Reward: 100 experience, 50 points
Title: Madman
Requirements: Win three duels in a row
Reward: 100 experience
Title: On the lamb
Requirements: Win five duels in a row
Reward: 150 experience
Title: Serial killer
Requirements: Win nine duels in a row
Reward: 300 experience, 50 points
Title: Terminator
Requirements: Win fifteen duels in a row
Reward: 500 experience, 2 cubelets
Title: Liquidator
Requirements: Win twenty-five duels in a row
Reward: 750 experience, 100 points, 3 cubelets
Title: Town takeover
Requirements: Complete all Duels achievements (Exclusives 1,000 & 10,000 wins)
Reward: 5,000 experience, 500 points, 5 cubelets
FFA
Title: Lady Death
Requirements: Kill five-hundred players with the Archer kit
Reward: 500 experience, 2 cubelets
Title: Dark magic
Requirements: Kill five-hundred players with the Wizard kit
Reward: 500 experience, 2 cubelets
Title: Phalanx
Requirements: Kill five-hundred players with the Beast kit
Reward: 500 experience, 2 cubelets
Title: Legionary
Requirements: Kill five-hundred players with the Swordsman kit
Reward: 500 experience, 2 cubelets
Title: Samurai
Requirements: Kill five-hundred players with the Ninja kit
Reward: 500 experience, 2 cubelets
Title: Centurion
Requirements: Kill one-hundred players with every kit
Reward: 500 experience
Title: Praetorian
Requirements: Kill five-hundred players with every kit
Reward: 2,500 experience, 5 cubelets
Title: Pentagon
Requirements: Kill five of each kit in one life
Reward: 5 experience, 5 points, 5 cubelets
Title: War Glutton
Requirements: Kill five players in rapid succession
Reward: 150 experience
Title: Regicide
Requirements: Kill a player who is on a fifty+ killstreak
Reward: 300 experience, 100 points
Title: Dethroned
Requirements: Die on a fifty+ killstreak
Reward: 50 experience, 10 points, 10 more points for sympathy
Title: Personified shield
Requirements: Take over 300 hearts of damage in one life
Reward: 500 experience
Title: Tyrant
Requirements: Complete all FFA achievements (Excludes 10,000 and 100,000 kills)
Reward: 5,000 experience, 500 points, 5 cubelets
Title: Lady Death
Requirements: Kill five-hundred players with the Archer kit
Reward: 500 experience, 2 cubelets
Title: Dark magic
Requirements: Kill five-hundred players with the Wizard kit
Reward: 500 experience, 2 cubelets
Title: Phalanx
Requirements: Kill five-hundred players with the Beast kit
Reward: 500 experience, 2 cubelets
Title: Legionary
Requirements: Kill five-hundred players with the Swordsman kit
Reward: 500 experience, 2 cubelets
Title: Samurai
Requirements: Kill five-hundred players with the Ninja kit
Reward: 500 experience, 2 cubelets
Title: Centurion
Requirements: Kill one-hundred players with every kit
Reward: 500 experience
Title: Praetorian
Requirements: Kill five-hundred players with every kit
Reward: 2,500 experience, 5 cubelets
Title: Pentagon
Requirements: Kill five of each kit in one life
Reward: 5 experience, 5 points, 5 cubelets
Title: War Glutton
Requirements: Kill five players in rapid succession
Reward: 150 experience
Title: Regicide
Requirements: Kill a player who is on a fifty+ killstreak
Reward: 300 experience, 100 points
Title: Dethroned
Requirements: Die on a fifty+ killstreak
Reward: 50 experience, 10 points, 10 more points for sympathy
Title: Personified shield
Requirements: Take over 300 hearts of damage in one life
Reward: 500 experience
Title: Tyrant
Requirements: Complete all FFA achievements (Excludes 10,000 and 100,000 kills)
Reward: 5,000 experience, 500 points, 5 cubelets
Assassins
Title: Leap of Faith
Requirements: Dive from the top of the pyramid to the ground (or equivalent distance) and survive
Reward: 150 experience, 30 points
Title: Quick kill
Requirements: Kill a player within seven seconds of spawning
Reward: 100 experience
Title: Quick death
Requirements: Die within seven seconds of spawning
Reward: 10 experience, 5 points
Title: Shoddy AI
Requirements: Kill five players in rapid succession
Reward: 150 experience
Title: Dirty work
Requirements: Turn in a bounty
Reward: 100 experience
Title: Getting the job done
Requirements: Turn in ten bounties
Reward: 200 experience
Title: Reputable assassin
Requirements: Turn in fifty bounties
Reward: 300 experience, 150 points, 1 cubelet
Title: Mob boss
Requirements: Turn in one-hundred bounties
Reward: 500 experience, 150 points, 3 cubelets
Title: Kingpin
Requirements: Turn in 250 bounties
Reward: 1,000 experience, 350 points, 5 cubelets
Title: Underworld
Requirements: Complete all Assassins achievements (Excludes 100 & 250 bounty claims)
Reward: 5,000 experience, 500 points, 5 cubelets
Title: Leap of Faith
Requirements: Dive from the top of the pyramid to the ground (or equivalent distance) and survive
Reward: 150 experience, 30 points
Title: Quick kill
Requirements: Kill a player within seven seconds of spawning
Reward: 100 experience
Title: Quick death
Requirements: Die within seven seconds of spawning
Reward: 10 experience, 5 points
Title: Shoddy AI
Requirements: Kill five players in rapid succession
Reward: 150 experience
Title: Dirty work
Requirements: Turn in a bounty
Reward: 100 experience
Title: Getting the job done
Requirements: Turn in ten bounties
Reward: 200 experience
Title: Reputable assassin
Requirements: Turn in fifty bounties
Reward: 300 experience, 150 points, 1 cubelet
Title: Mob boss
Requirements: Turn in one-hundred bounties
Reward: 500 experience, 150 points, 3 cubelets
Title: Kingpin
Requirements: Turn in 250 bounties
Reward: 1,000 experience, 350 points, 5 cubelets
Title: Underworld
Requirements: Complete all Assassins achievements (Excludes 100 & 250 bounty claims)
Reward: 5,000 experience, 500 points, 5 cubelets
Arcade
Title: Super soldier
Requirements: Acquire every power-up in one game of Paintball
Reward: 150 experience
Title: Ka-BOOM!
Requirements: Get two kills in one rocket in Paintball
Reward: 150 experience
Title: Can this thing get any faster?
Requirements: Pick up a Machine Gun power-up in Shotgun/Machine mode in Paintball
Reward: 50 experience
Title: Flexible loadout
Requirements: Kill one player with three different weapons in one game of Paintball
Reward: 200 experience, 50 points
Title: God of War
Requirements: Complete all Paintball achievements (Excludes 1,000 & 10,000 wins)
Reward: 5,000 experience, 500 points, 5 cubelets
Title: Speed greed
Requirements: Pick up seven Speed power-ups in one game of Spleef
Reward: 170 experience, 35 points
Title: Snow's end
Requirements: Break five-hundred blocks in one game of Spleef
Reward: 200 experience, 50 points
Title: Egg-splosive
Requirements: Land a direct hit with a big egg in Spleef
Reward: 100 experience, 20 points
Title: Can't stop!
Requirements: Don't stop sprinting in Spleef
Reward: 200 experience, 1 cubelet
Title: You're doing it wrong
Requirements: Land twenty direct hits with a bow in one game of Spleef
Reward: 50 experience
Title: The thaw
Requirements: Complete every Spleef achievement (Excluding 1,000 & 10,000 wins)
Reward: 5,000 experience, 500 points, 5 cubelets
Title: Super soldier
Requirements: Acquire every power-up in one game of Paintball
Reward: 150 experience
Title: Ka-BOOM!
Requirements: Get two kills in one rocket in Paintball
Reward: 150 experience
Title: Can this thing get any faster?
Requirements: Pick up a Machine Gun power-up in Shotgun/Machine mode in Paintball
Reward: 50 experience
Title: Flexible loadout
Requirements: Kill one player with three different weapons in one game of Paintball
Reward: 200 experience, 50 points
Title: God of War
Requirements: Complete all Paintball achievements (Excludes 1,000 & 10,000 wins)
Reward: 5,000 experience, 500 points, 5 cubelets
Title: Speed greed
Requirements: Pick up seven Speed power-ups in one game of Spleef
Reward: 170 experience, 35 points
Title: Snow's end
Requirements: Break five-hundred blocks in one game of Spleef
Reward: 200 experience, 50 points
Title: Egg-splosive
Requirements: Land a direct hit with a big egg in Spleef
Reward: 100 experience, 20 points
Title: Can't stop!
Requirements: Don't stop sprinting in Spleef
Reward: 200 experience, 1 cubelet
Title: You're doing it wrong
Requirements: Land twenty direct hits with a bow in one game of Spleef
Reward: 50 experience
Title: The thaw
Requirements: Complete every Spleef achievement (Excluding 1,000 & 10,000 wins)
Reward: 5,000 experience, 500 points, 5 cubelets
Snowman Survival
Title: Freeze!
Requirements: Stun three snow people in one game
Reward: 100 experience, 50 points
Title: The thaw
Requirements: Survive an entire game without using any power-ups
Reward: 200 experience, 1 cubelet
Title: Cold War
Requirements: Kill at least five survivors in one game
Reward: 250 experience
Title: Collectathon
Requirements: Collect at least one of every power-up in one game
Reward: 100 experience, 20 points
Title: Snow sniper
Requirements: Kill a survivor from at least forty blocks away
Reward: 150 experience, 50 points
Title: Torch-bearer
Requirements: Last at least two minutes as the only survivor
Reward: 150 experience, 50 points
Title: Ghost
Requirements: Activate a Blink power-up whilst your decoy is still active
Reward: 100 experience
Title: Wearing a scarf and gloves
Requirements: Survive one game of Snowman Survival
Reward: 100 experience
Title: Dashing through the snow
Requirements: Survive ten games of Snowman Survival
Reward: 120 experience, 1 cubelet
Title: The worst is yet to come...
Requirements: Survive one-hundred games of Snowman Survival
Reward: 1,000 experience, 100 points, 2 cubelets
Title: Deep Freeze
Requirements: Survive one-thousand games of Snowman Survival
Reward: 5,000 experience, 500 points, 3 cubelets
Title: Outlasted winter
Requirements: Survive ten-thousand games of Snowman Survival
Reward: 7,500 experience, 1,000 points, 10 cubelets
Title: Winter's end
Requirements: Complete every Snowman Survival achievement (Excludes 1,000 & 10,000 wins)
Reward: 5,000 experience, 500 points, 5 cubelets
Title: Freeze!
Requirements: Stun three snow people in one game
Reward: 100 experience, 50 points
Title: The thaw
Requirements: Survive an entire game without using any power-ups
Reward: 200 experience, 1 cubelet
Title: Cold War
Requirements: Kill at least five survivors in one game
Reward: 250 experience
Title: Collectathon
Requirements: Collect at least one of every power-up in one game
Reward: 100 experience, 20 points
Title: Snow sniper
Requirements: Kill a survivor from at least forty blocks away
Reward: 150 experience, 50 points
Title: Torch-bearer
Requirements: Last at least two minutes as the only survivor
Reward: 150 experience, 50 points
Title: Ghost
Requirements: Activate a Blink power-up whilst your decoy is still active
Reward: 100 experience
Title: Wearing a scarf and gloves
Requirements: Survive one game of Snowman Survival
Reward: 100 experience
Title: Dashing through the snow
Requirements: Survive ten games of Snowman Survival
Reward: 120 experience, 1 cubelet
Title: The worst is yet to come...
Requirements: Survive one-hundred games of Snowman Survival
Reward: 1,000 experience, 100 points, 2 cubelets
Title: Deep Freeze
Requirements: Survive one-thousand games of Snowman Survival
Reward: 5,000 experience, 500 points, 3 cubelets
Title: Outlasted winter
Requirements: Survive ten-thousand games of Snowman Survival
Reward: 7,500 experience, 1,000 points, 10 cubelets
Title: Winter's end
Requirements: Complete every Snowman Survival achievement (Excludes 1,000 & 10,000 wins)
Reward: 5,000 experience, 500 points, 5 cubelets
Completionist
Title: Milestone #1
Requirements: Reach level 25
Reward: 1 cubelet, 500 vanity points
Title: Milestone #2
Requirements: Reach level 50
Reward: 5 cubelets, 5,000 vanity points
Title: Milestone #3
Requirements: Reach level 75
Reward: 10 cubelets, 10,000 vanity points
Title: Milestone #4
Requirements: Reach level 100
Reward: Speed II in lobbies, Glow effect in lobbies, 20,000 points, 20,000 vanity points
Title: Milestone #5
Requirements: Reach level 150
Reward: Speed III in lobbies, 20,000 points, 20,000 vanity points
Title: Milestone #6
Requirements: Reach level 200
Reward: Speed IV in lobbies, Teleporter Bow in lobbies , 50 cubelets, 50,000 vanity points
Title: Final Boss
Requirements: Earn all achievements in all games (excludes win achievements)
Reward: 50,000 experience, 30,000 points, 50 cubelets, 50,000 vanity points
Title: Completionist
Requirements: Complete 'Milestone #6' and 'Final Boss'
Reward: 50,000 experience, 30,000 points, 100 cubelets, 50,000 vanity points, IGN listed on the 'Completionists' page on the forums
Other reward: First person to complete it will have an area on the server dedicated to them
Title: Milestone #1
Requirements: Reach level 25
Reward: 1 cubelet, 500 vanity points
Title: Milestone #2
Requirements: Reach level 50
Reward: 5 cubelets, 5,000 vanity points
Title: Milestone #3
Requirements: Reach level 75
Reward: 10 cubelets, 10,000 vanity points
Title: Milestone #4
Requirements: Reach level 100
Reward: Speed II in lobbies, Glow effect in lobbies, 20,000 points, 20,000 vanity points
Title: Milestone #5
Requirements: Reach level 150
Reward: Speed III in lobbies, 20,000 points, 20,000 vanity points
Title: Milestone #6
Requirements: Reach level 200
Reward: Speed IV in lobbies, Teleporter Bow in lobbies , 50 cubelets, 50,000 vanity points
Title: Final Boss
Requirements: Earn all achievements in all games (excludes win achievements)
Reward: 50,000 experience, 30,000 points, 50 cubelets, 50,000 vanity points
Title: Completionist
Requirements: Complete 'Milestone #6' and 'Final Boss'
Reward: 50,000 experience, 30,000 points, 100 cubelets, 50,000 vanity points, IGN listed on the 'Completionists' page on the forums
Other reward: First person to complete it will have an area on the server dedicated to them
When you photograph the photographer
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Total: 99 achievements
May add more later on down the line, but considering how time-consuming this was, I wouldn't count on it.
Title: 100 achievements
Requirements: Suggest 100 new achievements on the forums
Reward: Owner rank
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