I personally want the Nether to be an extremely rare underground overworld biome. That would be so cool. And maybe make villages spawn in badlands and look like Wild West towns.
Maybe a new ocean biome that stretches right down to bedrock and has awesome ores everywhere. Ocean Monuments should definitely have loot chests.
I really, really want a gigantic cave that is like 40 blocks high and 60-something blocks wide, just because that would be cool to build an underground city in.
Oh, one more thing. They need to make it possible to choose what you want to spawn in a world, so you can shut off creeper and bat spawning, up gold ore spawning, or even make the Nether more challenging.
