They pretty much wrapped up the Emerald Dream in the book (Stormrage) and Cataclysm. They might use Azshara as one of the first raid tiers kind of how Vashj was in TBC, and Nzoth later on. We all know KJ will be one of the raid bosses (because he didn't die in the Sunwell), and Velen might have some involvement there (they were great friends at some point). And then there's Sargeras, whom I think will be the End Boss of the next Expansion. But if he dies then what's left? He's the biggest villain of them all, bigger than any Old God. They might make an Old God themed expansion after this, but they would need to finish up the lore for that as Dave Kosak said. Who knows what the future lies in store for Azeroth.