Argus as an expansion isn't happening.That's a really interesting picture you are describing. I myself wish for something along those lines.
As long as they can properly explain non-entirely-corrupted environments it would be a good set-up for an expansion. Mac'Aree and surroundings are obviously going to be under heavy Legion influence, with grandiose Eredar architecture here and there, with little touches of fel and Legion influence even within the city, but preserving the Eredar legacy.
The Army of Light could conceivably be in control of certain areas that may be in a somewhat state of purification, even if we're talking about underground or isolated vales and mountains where guerrilla warfare might prove to be a problem to large Legion armies.
I think Argus would open up a whole new array of opportunities regarding world physics and natural landscapes, same as Outland did back in the day. But got to be really careful with overly "fel" environments as you mention, since Legion did not really put an incredible amount of focus on this apart from the Broken Shore and certain world areas, but still proved to cause fel fatigue for some.