Ethereals and Mechagnomes for Alliance are my top picks. I don't play Horde, so no real specific picks for them.
I'd also love to see Ogres, Naga, Broken, Saurok, Wildhammer Dwarves, Jinyu, Hozen, Vrykul and some new original races become playable someday. The sky's the limit with allied races, they could put in just about anything, so I hope they go as crazy with it as I would.