And this is the right way. Do you want fight only with demons every tier? Nighthold have 3 demon bosses (Tichondrius, Krosus, Guldan), Tomb around 4 (or 5? I don't think we know what 9th boss will be) + demon only mini-zone + demon dungeon. So Legion have already big presence before Argus. At least demons are more diverse than orcs.
I hope that Argus will have different look than Broken Shore, I don't know why, but I always imagined this like white metropolitan city of Mac'Aree with violet environment.