I personally think that if there is ever a good chance to get us an Argus expansion, it would be after Legion. The real question is, if that comes to pass, what we will do on Argus if we killed Kil'jaeden in Legion?
The only thing that comes to my mind is that Argus will be a base to build the Army of Light in the face of the Void Lords down the road. Hell, if that comes to pass -theoretically-, then this would also be a good chance to introduce the ethereals as playable since people seem to want them all the time. They can be the only race that could go mercenary and join both factions, too.