Just for fun, more:
Vote for Superman
Enable flight. Have to make sure x-y-z travel ranges are limited properly.
Vote for Death Santa
Every time a player dies, every chest has a 50% chance of gaining a random diamond item (including axes, shovels, raw diamonds) and a 50% chance of a lump of coal.
Vote for The Last Of Us
Every player starts with a cat on their island. Let's think of the cat as "Ellie". If the cat dies, the player dies too (and vice-versa).
Vote for Grenades
All snow (+eggs?, +arrows?) causes grenade type effects like those seen in the paintball mini-game.
Vote for the Tower of Doom
Dead players re-spawn in a tower in the middle with a bow and 10 arrows (or more?). They can't leave the middle, but...
Vote for the Charge of the Light Brigade
All islands start with a saddled horse. Not that I think they will be really useful, it's more of a funny concept. Possibly good when combined with starting bridges.
Vote for Night Of The Living Dead
A few seconds after spawn, a zombie mob spawner is placed where the glass starting prison is on every island. Must be night time :P Spawner must be low enough so that the zombies don't die on impact. Remember they (by default) only activate when players are near, which is what you want.
Vote for The Navy
A lake is put surrounding/between islands. Smart people build boats.
Vote for Hallucinations
Diamond armored/sworded (non-player) Steves appear and disappear on the map randomly every few minutes. If they are hit, they explode like a creeper. I guess the simplest way to do this is to reskin creepers as Steves, actually.
(Again, I know these are not trivial to implement. I'm throwing out so many options so only the best can be implemented).
