Nice work... reminds me a bit of Hyboria from Conan.

Would be intresting to give the mod a slight bit fantasy. Why not play with that lore a bit?
And for the question with the invasion. I think one of the biggest fears of the time was the hungerdeath... a bit of misfortune could kill hole tribes. So how about hungercorpses? you could use the native undead mesh (the one with flesh xD) and give it a new texture.. Leatherlike skin.. dirty from the months of wandering.. And pitch black holes where the eyes would be. A big horde of canibalistic undeads that inbodyment the biggest fears of the time.
so big hordes of hunger and frozen corpses.
Another idea would be ape like orcs. Think about bands of barbaric, nearly naked humanlike beasts... with big and spiky fur. if you use planes that stick out of the body you could create that effect. But i would not make them orclike broad. More like skinny humans.
And how about on smaler faction that got a bit further in the timeline? With a bit abstract copper age stuff. The argonians from Conan are an good exemple. Maybe a bit more babaric.