I think Ethereals do have serious potential as a playable race. If Blizzard wants to go the neutral race route again, then perhaps we could have one faction fall in with Alleria and Turalyon (regardless of wether they are on K'aresh or not, Ethereals do get around) and join the Alliance while another ends up getting in contact with the Blood Elves and joins the Horde. If not a neutral race, then I would probably peg the Ethereals as Alliance-exclusive since they could give the Alliance a more commerce centric race, and the Horde could perhaps get a splinter faction of a race normally in service to the Legion (there are about a dozen possibilities, but I'd go with Nathrezim.
With the Nathrezim we could have an amusing inverse of us accidentally digging up ancient horrors. Basically the Burning Legion would go digging in the wrong place and end up accidentally unleashing the "pure" Nathrezim from magical stasis. They would naturally be attracted to a faction that had thrown off corruption in the past, and would fall in line with the Horde should they make contact.