Good luck with the mod. It may be better if you do not add extra factions for now, the conflict is one between the Romans, Persians and Arabs, though you could have a couple of units of Armenian mercenaries or something from around that region. It would be interesting to have the Sassanid Horse Temple and/or the True Cross featuring somehow in the mod. Perhaps their loss should lower diplomatic relations or cause a war but maybe something more like a morale boost to the enemy faction. Also presumably there will be a rebel faction and hopefully some interesting rebel units for the Armenian, Ghassanid and Lakhmid region but preferably no factions as such. Conquering this part of the world as Serbs, Armenians, Ghassanids, Hephthalites, etc sounds mad. I would also throw in the idea of asking for units/skins from other mods for now (e.g. from the Great Conflicts) and later make new ones if you have the inclination.
+rep