Parsig

I think I'm everything on that list except Persian and Iberian

Parthian sub factions could be different family dynasties, like the Surens, Mihrans, and Karens, to reflect the feudal nature of the empire. Each could have special units and bonuses based off of the region they historically held power in. Surens = Saka light cavalry, Kushan Infantry, Gondophares as a hero. Mihrans = Strong Daylamite infantry, Bahram Chobin as a hero Karens = defensive bonuses, Hunnic and Median units Sassanids (might as well throw them in as a possiblity) = Strong priests and heavy cavalry, Kartir and Khosrow as heroes