I would say that the most likely next expansion is something with the seas. The Maelstrom, the south seas, and a glimpse into the Emerald Dream. That's the most likely next expansion, and that's what most people will say.

Personally, what I would like to see is for Blizzard to take another shot at the sci-fi elements they tried with The Burning Crusade. Outland suffered from being too techy, too bright. Even if you liked the wow powerleveling out-of-this-world crystalline decor and space-age tech making an entrance in Warcraft, the degree at which it was used made you sick of it really fast. Psychedelic crystals were pretty... for awhile. Outland, they were in half of the zones. That's far too much, especially when all of those areas see heavy traffic, like 3 instances and a raid zone, for example.

I think that they could take the otherworldly parts of The Burning Crusade, stick it in a blender with Wrath's gothic fantasy, and come out with something truly awesome. They did originally want us to visit multiple worlds in The Burning Crusade, so I think they'll want to explore that again.

Come on, if it's done right, you can't tell me taking the fight to the Burning Legion and invading their worlds for once isn't an attractive prospect. I think that has the potential for expanding the Warcraft universe wow powerleveling moreso than just bopping around in the Maelstrom, too. Learning more about the Legion beyond what we've seen on Azeroth, meeting other races, developing the story right before our eyes rather than just dealing with leftover baggage from the RTS games. Lots of potential.