What about the ongoing legendary questline with Wrathion? Wrathion has been referencing the return of the Burning Legion in much of his quest text. Does that mean demons will be descending upon Azeroth in the next expansion? Will we finally get to see Sargeras?
Also, you wouldn't want to forget Queen Azshara. You would think we'd have seen her and the city Nazjatar in the Cataclysm expansion, but so far she's only made minor or indirect appearances. Azshara is the perfect accompaniment to some recent fan speculation about the naga being the next possible playable race. Unfortunately, Ghostcrawler pretty much shot that down by pointing out the naga's lack of legs would make it difficult to design armor for them.
And, of course, there's always ... Trolls? Wait, really? More trolls? You wouldn't think it, but in the same interview where Ghostcrawler killed our naga dreams, he said that trolls were an option the game lore could still explore in future content. Huh ... Go figure.
So, what do you think the next WoW expansion will be?