We know that the Horde will probably get ogres at some point, but what could be the next playable Alliance race?
High elves are extremely unlikely considering that lore-wise they're a minority of a minority and aren't really different enough from blood elves.
WoD would have been a great time to introduce arrakoa as playable, but that time has passed.
Naga and dryads/keepers would basically break established game mechanics and ultimately end up as a conceptual clusterfuck for Blizz to try and implement.
So, here's my candidate for a possible playable race:
Dragonman or another Dragon/Reptilian race
While drakonids and dragonspawns present problems that prevent them from being playable, another humanoid dragon-like race is feasible. You have dragonmen which are already in-game and with a few tweaks they could easily fit within the Alliance.