Hasn't been heavily rumored that the next expansion is going to be focused on Azshara, since Blizzard recently trademarked "The Eye of Azshara"?
Anyway, the Azshara-focused expansion is what I've been waiting for since I took an actually interest in Warcraft lore. Queen Azshara is easily one of the most interesting entities in lore. The very form Azeroth has taken, and pretty much every major conflict on the planet, can be traced back to her original sin. And she's been present this entire time, constantly plotting, constantly maneuvering in ways that can be felt by the Alliance and Horde, but never really playing her hand. Also, by my calculation now that the Aspects have been de-aspected (and maybe even before that), Queen Azshara is easily the single most powerful being on Azeroth, if we're going to compare feats accomplished.
She's too iconic to not get her own expansion, and lordy I hope it's the next one.
That could just be a Heartstone expansion.