It does go out with a whimper does it not? How about if not only did Hell Terrian spread across the globe. But barb spawns were replaced by barbarian demon spawns? Being the end of the world, seems appropaite. Perhaps even introdue a new demon unit or two, only available to the barbs? These wouldn't be controlable by Hyborem. Say their non alighned creatures of hell, and when he grows too powerful, just like the other barbs, they will try to pull him down.
For an extra twist, include a few angel barb spawns. Determined fighters that care for nothing but the war their about, rouge elements of Basium's host who answer to no leader. Then it might need tweaking, be odd of angels stacked with demons to attack your city.. but theme wise, it could be intersting.