I doubt it would be any of them.
Arrakoa? These for me are the most "probable" since they are Outlandish, and its possible (even probably imo) that next exp will be Burning Legion based, and the arakkoa are outlandish. (While the next exp might be legion based, i doubt we will be going back to Outland though. Rather a new place than a revanped Outland imo.)
They would however imo be limited with class choices, and they are a bit too bestial and uncivilized to be a playable race, imho.
Etheals: Armor problems. Ethereals are what their name implies. ETHEREAL. Not really solid Hard to get armor on them.
High elves: Too similar to blood elves.
Naga: What? Naga are, unlike the arrakoa, ALL EVIL, no i just dont believe in a good naga faction. And of course the obvious armor and animation problems. Cmon, they are serpentine. NO LEGS. How would they even ride. Speaking of riding, arakkoa on mounts let alone flying mounts would be strange, strange sight.
So i think the options are quite flawed. If there is going to be a new race, it is more likely to be someting new and more humanoid than this, like the Pandaren were. Also, i have the same opinion on Ogres for Horde. Ugh. Just no.