Honestly, I don't know. I suppose whatever comes along that strikes my fancy.
This is something I've thought a lot about recently, what with the general consensus about Cata being "meh, been there, done that," and all.
I think most of us can agree that WoW has peaked, one only has to look at many of the remarks on this very board to see that. That said, I'm probably not quitting any time soon - I said "peaked," not "keeled over and died of a massive stroke." WoW isn't dead yet, and there's still a lot of fun to be had. But I think we are on the downward slope of things.
As for what comes next, I don't really know. I've always enjoyed online games, and Call of Duty-style online doesn't strike me as actual online. My next big game is going to be a PC game, I think, likely an MMO. But what that is remains to be seen. I can guarantee it won't be Rift (nothing against the game, but I know it's not for me), but I can't guarantee what it will be.