I answered: 'different troop trees'. But even that I think is too much. I'd rather have a couple of unique units for each Empire faction, as others have said. This would represent the culture of the lands they occupy and what auxiliaries would be available. So maybe:
- Southern Empire - Horse archers, since they encounter the Khuzaits and Asserai a lot
- Northern Empire - Shock infantry, because of all the Northern Barbarians in the region
- Western Empire - Dunno, they already have heavy cavalry and crossbows. Maybe limit crossbows to the Western Empire only and make them more elite like the Vlandian version.