They're on one of the major worlds dominated by the Legion. We're going to rescue them, or join up with them, in the final expansion. Make it happen!
I'd like to see them return. What would be "interesting" is if they returned "as demons"; but, wholly uncorrupted - hence why they didn't want to return in the first place.
Or - it'd be the Signal of the High Elven race being playable by Alliance.