There was a Blizzard quote a while ago, basically said they were working on WotLK slightly prior to the release of Burning Crusade.

So there's a good chance whatever the next one will be, they're already working on it possibly prior to the release of Cataclysm.

They said there wouldn't be a heroic class in Cataclysm, they don't want a heroic class in each expansion. Not sure if that means there might be in the next one.