I'd like something new, like Arctic.
Anyway, I think that
biome is a wrong word - because there's no Biome on Earth comparable to Hellfire Peninsula or Argus.
I personally don't want any earth biomes, the most beautiful zones in WoW were the ones which don't appear in real life: Blades Edge Mountains, Hellfire Peninsula, Netherstorm, Deepholm etc. Whenever I was going through these I felt like in a real fantasy world.
But if I have to choose then I'd say: Arctic, just plain snow/ice. No trees, no mountains, just plain snow/ice with caves(Forstfire Ridge or wotlk zones wasn't that representative):
But if I don't, then I'd like to see "full fantasy" places - like realm of shadows/ghosts/death:
Flying isles(bigger than the ones from Nagrand):
etc.
It's a fantasy game. I want to see spectacular things, not some lame realistic forest for low level characters.