Well, Bliz is staying mum, but they are working on it.
…the “general layout” for number five is already in place.
“They’re [the] lore creators. They’re the ones who drive what the world looks like today and what it’ll be like tomorrow and beyond,” said Sims. “I think the future of World of Warcraft continues to be bright and I think Mists of Pandaria is going to breathe huge life into what I believe is the best MMO out there.”
So, in some mystical universe, Blizzard puts you in charge of whatever comes after MoP. What’s gonna be in it? New race(s?) New Class(es?) New evils to stomp out?
There’s another version of this post, with many people’s ideas for a new expansion. See it here: The Next World of Warcraft Expansion