These are just my ideas for what needs to be in minecraft, if you have a idea why don't you comment? don't like one of my ideas? comment about it! or if you think my ideas are super awesome, why don't you diamond it? or better yet tweet this blog to Jeb, or make a mod of it! (Go ahead I don't mind.)
1: Redstone activated jukebox's. For adventure map purposes, I would love it if was able to activate music by having a player walk over a tripwire, it would be a dream come true.
2: Librarian villagers should sell pre-written in books. it would be even better if they where player written books. like mojang could have a book writing contest for minecraft and the winners would get their books put in the game.
3: On off switches for breakable and placable blocks in adventure mode, This feature is probably going to get added soon but for now it's on this list.
4: mobs that are actually scary. (I have some ideas for some mobs like this that I would like to make into a future mod. I just need someone who knows how to code...)
5: Enchantment crafting. Imagine being able to increase the likelihood of certain enchantments or unlocking new enchantments by placing in a craft-able item, like a talisman or rune, then placing that item in a new slot of the enchantment table, then choosing the level of that enchantment.
6: Spells. some for building, some for fighting all of them should be awesome.
7: Useless blocks that look nice, Kinda like the chiseled stone brick or the sandstone blocks.
8: Wool half-slabs, actually i want half-slabs for EVERY BLOCK.
9: Stained glass. basically glass blocks with a tint that you would make by combining glass with dye.
10: A redstone clock item. kinda like the repeaters... but a clock. (i think theres something like this in the redpower mod.)
11: Colored torches, in other words torches with color, you know, for mood lighting.
12: Anti-torches (torches that remove light). I think these would be fun for dungeon maps.
13: A invisible light block with brightness and color settings. for building and adventure maps.
14: Desert villages that aren't a just re-color. (Seriously Mojang?)
15: A gravity potion. kinda like what they had in Terraria. (It swapped the gravity on the player.) I think this would make for some fun traps and it could be a interesting gameplay element in adventure maps.
16: A flight potion. Flying with a time limit!
17: Randomly generated dungeons. Kinda like the mineshafts and the ruin's but bigger and more deadly.
18: Spike blocks. It would be even cooler if they could get pushed by pistons!
19: Iron armor (or anything made of iron) should heat up when near/in lava/fire. it just makes sense.
20: (Last one) A way to change spawner types (Creative feature only obviously)