Going back to Draenor seems like a legitimate topic for an expansion. Focus outward on the Burning Legion, bring back Alleria and Turalyon.
The time travel part of things seems a little more sketchy. I'd be more inclined to believe that there are a couple Caverns of Time dungeons focusing on Grom Hellscream or Ner'zhul and the rise of the Horde.
The biggest problem with a time travel expac is how would you explain all these different races showing up? A Pandaren in Outlands? The Caverns of Time handles it by putting us in disguise as the opposite faction as appropriate. But while that's fine (and even fun) for a single instance, having your character be a different model for an entire expac does not sound appealing.
Still, we'll see what happens at Blizzcon. Personally, I'm still holding out for Corgis Unleashed!