Reskins for unit models. Or just use different models from ingame model library.

i think it would be cool if you added an easter egg that would make a modern nation appear with modern ideals and tech, and etc, into the game (not too big tho, maybe like luxembourg or cyprus). to prevent them from destroying the entire game a shield would be built around luxembourg that doesn't allow anything in or out, and that ships will sink in cypriot coast waters.

Definately events that pop up the longer the game goes. These events should make new nations apear after a while (Poland,Hungary the Muslim Caliphate and some sort event around HRE)