Here's my Real Prediction: Azshara

Azshara, twisted queen of the Naga: Our next big challenge

Haha. I'm almost sorry to keep coming back to this issue, but I keep giving it more and more thought.

I think I'm ready to put down my bet: The major antagonist of the next expansion is going to be Azshara. I'm not sure that makes the whole thing and "undersea" expansion per se. There are plenty of creative ways to introduce different settings to a story. But that's what it will be: Time to finish the Naga threat.

The thought that occurred to me about an Azshara-based expansion: Blizzard originally planned two raids for tier 12 raiding, the Firelands and an Abyssal Maw raid. The undersea raid was tabled, since the devs were allegedly more "excited" about the Firelands. 

Abyssal Maw had been announced and it's cut from patch 4.2 was one of the things that has been a major, major letdown. But, reading between the lines, if Blizzard were actively contemplating a significant undersea expansion, it would make a lot of sense to save the ocean raid material when they know it will thematically fit into the story they have coming up.