Thank you for your work.
Orcs and goblins, dwarves, bretonnia, vampires etc. are not one single faction, too. But there are no 5 different orcish tribes.
Can't you turn some of those factions into rebels, not all empire factions need to be playable. Europa barbarorum had a different rebel tag for each city which represented the tribe that lived there. And each rebel city had full garrison. You can make the same thing here for some empire factions.