Playable races.. hmm. Well, taking into Velen's prophecy about the clash against Azeroth's might combined, in which the Horde and the Alliance are to team up against the Burning Legion once more, it is said that the Dragons also join the fray, to aid them. No, I'm not saying we'll get playable dragons. That would be silly.
But what if we got Drakonids?
Or Ethereals? They could both be like Pandaren - neutral to begin with, and then choose a side.
Or maybe get a Dragonsworn as a playable class, but that's derailing from the topic, which is playable races, not classes.
Still, I'm hoping this is indeed a WoW expansion that features The Legion, which has always been my favourite theme, and I can't wait for more details!