This thread is for people who want to post suggestions for either new biomes themselves or to add things to already existing biomes.
Now to get things started, I'd like to see an ice biome with Moulins.
A moulin or glacier mill is a roughly circular, vertical to nearly vertical well-like shaft within a glacier through which water enters from the surface.
I would like to see a wasteland. It would be a flat place with ugly grey or brown grass and nasty brown water. Also no trees. This biome would be perfect for building a ruined village afected by nuclear fallout. Also a rocky biome would be cool. It would be a biome with no grass blocks on it.
I'd like to see a little more variety of stuff in the existing biomes. Much has been said in other threads. Fruit trees, critters (like horses), etc. So for the forest biome could have apple trees (instead of oak dropping apples), jungle could have banana or orange trees. Swamp could have papyrus (to replace sugar cane for paper). Other items could be generated as well, like nightshade, etc.
Subterranean biome could be cool, with new fungi, mobs, plants (maybe flourescent moss or mushrooms that can be a new light source), and so forth.