During a Dev interview they outright confirmed 7.3 was Argus giving us a teaser image of the Argus we saw in the cinematic for 7.2
We are going to Argus in this expansion, the devs dont want to make an entire expansion around it im afraid.
And honestly I dont blame them, Argus would be a cool way to finish this Legion trilogy with WoD being the first and what ever Argus expansion could have been being the last, but I dont see it happening now. People are tired of demons, Legion and generally everything related to them.