suggestion for new faction

[SD1987]

I doubt this idea would work though. AI in Medieval 2 Total War isn't very sophisticated on the campaign map. Allies and vassals will declare war on you at random, usually one turn after the alliance/vassalage was made, so the Seleucid satrapy would be at war with the Seleucids as soon as you hit end turn at the start of the game.

This is totally untrue, you're thinking about RTW's horrible game-breaking AI. Medieval 2 did good things with campaign map AI and mods made it even better, did you ever play Stainless Steel? Campaign map diplomacy was pretty excellent.