The paint hasn't even dried (or, in some cases, been applied) on the Burning Crusade, but players have already begun speculating about the next expansion pack-- the one Blizzard said would come a year from now. It's not all completely unfounded-- a CM has hinted (ok, he was joking) at Uldum finally showing up, but this is the videogame industry, so if there's any chance at all that something might come out in the next century (Duke Nukem, anyone), players are talking about how cool it will be.
Besides the Uldum guess, a lot of players say Northrend and Arthas, but it's true that Blizzard has said a few times that that's another few expansions away. Personally, I'm throwing my hat in the "back to the seas" camp-- I think we'll see what's in the middle of the map, and maybe get some swimming mounts. Or maybe some new races? Maybe Murlocs or Naga will make a show as player characters eventually. Paladinsucks is in on the speculation, but as usually, they're only concerned about how their shiny Paladin selves will look.
Ok, ok, so we haven't even opened the plastic wrap on the Burning Crusade yet. Feel free to go back to running Hellfire Citadel and getting to 70. But know that as soon as a year from now (snicker), we could all be doing it all over again.