Here's the thing...
All the evidence seems to suggest that we'll face the burning legion again in the next expansion.
I would love this so much, and really missed the old-school Warcraft themes and story lines that come with that.
I hope to see Turalyon and Alleria, and I want the opportunity to see Argus. Something new.
But then I think of Vashj'ir. Love it or hate it, it is a special zone.
One could say it is my favourite zone in recent years.
It's something about the underwater theme, the music, and the fact that you have this vast, unknown world in front of you once you take the plunge that interests me. Imagine that x10. Imagine an expansion where you just went deeper and deeper into the ocean, saw a bunch of strange and absurd worlds down there - or pleasant little tropical islands by the surface. You'd obviously use that seahorse mount of yours, so moving around would be fine.
But anyway. What would be your preferred expansion? I'm really split down the middle on this one!