TBC = Blood elves and dranei
Wotlk = No new races, only death knights as new class.
Cataclysm = Worgens + Goblins, new race, not new class.
MOP = New race and class.
I appreciate they don't have to stick to a particular pattern, but through speculation we can "guess" that there won't be a new class, instead their MAY be a new race.
So, any thoughts on what this new race may be? Obviously it will have to fit with the expansion theme, which we don't know yet, however some people have rumoured that it will be something to do with the burn crusade - although we dont have any clues or evidence to suggest that, yet, it's just a guess.
My hope for a new race playable would be an etheral. I just think it would be fun to play an ethereal mage or rogue. I don't know what fraction they would belong too, perhaps neither (like monks) or they may introduce two races, but at the moment I have no guesses as to what the 2nd could be.