A tracker to see how many mobs killed/blocks have been broken (It'd be fun to have that info!)
Crouching, then crouching again to crawl or some new movement system
Shaders! (We've been promised two separate times for it and they've still not gotten it done)
More custom models for the player! Like Steve and Alex models, but more crazy than just arm size. Like how about a skin model with no head, and new animations and armor layouts to go with it? (you could texture it to be SpongeBob for example) Or one that's specially made for use with long hair? (Like a really long hat spot to texture, and it all be attached to the head?) Maybe one with just a slightly taller head so that you could make yourself look like a villager, or MegaMind or something lol More variation would be AWESOME!
