FFA Line Dash; This may be a little bit difficult due to the fact that there is a limited amount of colours for carpets, therefore using different blocks in the Minecraft realm may be needed to be used. I like the premise of it though, however the gameplay may be a bit finicky for someone who just wants to play some rounds of Line Dash due to shop campers, and the sheer amount of tails there will be.
Singleplayer Line Dash; YES! This sounds awesome :D +1 from me.
Whack-A-Mole; With this game, using a bow and arrow may be annoying at later stages, where I assume that it would get a faster paced. This makes you have to almost predict where the "moles" come out of, which may get confusing and just luck based. If it is a small grid for everyone, and they have to run around and whack the moles, then that sounds a little better.
Mob Arena; I reckon this could be an awesome thing to have next to PvP, or placed in FFA or Assassinations. You can type a command like "/mobarena" and you get teleported to an arena and kill mobs, progress through rounds and stuff. You can use the different kits in FFA, or use your collected gear in assassinations (armour durability can be broken down in the arena, however you will receive it back outside of it).
Quakecraft revamp; I'd love to see a good revamp of Quakecraft, respawning shouldn't be clicking on the respawn button though, because that causes issues with "lag" and whatnot.
Obstacle Course; With this, we would need to make sure that it isn't a copy of the whole "Wipeout" thing, and have Cubecraft twists in it. However an obstacle course sounds awesome. It reminds me of a progressive Minerware, which is cool.
Tron; Sounds awesome. Perhaps a voting option for Line Dash?
Party Improvements; 1 - This is why it may not be possible, however it may be cool. 2 - Sounds awesome.
Dropper; I remember when Dropper was going to get released in the old parkour lobby, but it never did </3
Anyways, I would like to see dropper, however it may be less arcade and more parkour based, which may not fit into the arcade section.
Overall, awesome thread Grim! (I didn't answer the TD one because I never play it. :P)