Well I think they need the Aether, I mean that could expand the game in a big way. Also It just makes sense to have it if there is the Nether, which is darkness and Aether is light.

I've heard though that they are going to be working on more LAP server resources to help it run better on 1.4 that information was found on mojang.com

I think there should be more options for your world like when your creating a world you can make it an rpg or a superflat (already in game) or a regular world or even a worlds thats just a void!

more ores mores rarer generating structures and more mobs

Definitely this:
http://www.minecraftforum.net/topic/943 ... ier-menus/

Mod API.

New ores like silver, platinum, mythril, adamant, onyx etc.