This is what should be added in minecraft

*piston that extends 2 blocks out
*a block that rotates blocks around it
*redstone dust that can be moved by pistons
*item detector (you set the item that it detects)
*2 seated mine-carts/boats
*Heat bar (like hunger bar)
*bacon
*balloons
*mob love block (one for each mob, if a mob sees there block they will walk towards it, they cant see them thru blocks and 15 blocks away is the limit they don't need to be looking at it,i can be behind them or to the side of them)
*chair
*player sensor
*bird mob
*food transporter (food travels thru it)
*fish mob
*quick sand
*camera
*key/lockable door
*emerald armor/tools/sword
*diamond arrows/shears
*golden bow
*country biome (thorns,bush,cow poo,gravel rivers)
*fruits + they drop from leaves
*lizard mob
*lightning rod/conductor (lightning is very likely to strike it or near it.if that happens the rod will emit a redstone pulse)
*npc city's/citizens (big houses,beds,redstone,roads)
*throwable TNT
*capes (added to the skins)
*acid
*taming potion (tames any mob)
*chest/furnace that follows you
*cuts ,wounds and risk of losing body parts if they are knocked off or get caught on something
*rings
*readable books (get one in some chests ,villager trading ,right clicking on bookshelf)
*sticky block
*mud
*wind (blows you,mobs,floating blocks and trees around)
*pipes (1x1-3x3 transport lava, water, redstone power, particles)

what others suggested:

*guns by Miner90000
*owls by Crazyowl693
*Christmas trees (lights on it and it is only at Christmas) by ballbreaker1
*a minion by Mine Trustin
*quarry's by Mine Trustin
*lapses ore will drop a lapses diamond by parkour master
*silver ore by orionaboss
*horses lions tigers bears and sharks by orionaboss
what others think about the suggestions:

*none yet sorry
(but you just have to comment what what you think about the suggestions just start the comment with "w.i.t.")

most suggestions:
1st: Mine Trustin 2 ideas
2nd: ballbreaker1 1 idea
2nd: parkour master 1 idea
2nd: Miner90000 1 idea
2nd: Crazyowl693 1 idea
(challenge end date:about 20/2/2013)
(i am 1st but i don't count)

(you could also make some of these into mods)
(if you make any of these into a mod plz notify me and give me a link to it)
(post in the comments what you suggest and i will add it with your name there)
More coming soon.