While I think it's pretty certain that the next game expansion will add additional levels to the game, there's a reasonably sized petition making the rounds on the forums asking Blizzard not to add more levels
. The argument, of course, is that we could see a repeat of the Burning Crusade -- all raid content is abandoned as the first quest rewards from new zones equal it while the reputations you spent so much time grinding up now only offers useless, low level rewards. But though I understand the concerns of players railing against new levels, I think they're going to come, anyway -- Blizzard has to give the players who have already burned through Burning Crusade content something new and interesting to do. However, perhaps they've learned a bit from the last expansion... only time will tell.
But what do you think? Does the game need new levels, or does that just destroy the things we've worked so hard to achieve?