Maybe there could be an end game Crisis like mechnic where the Romans show up.
Historically they showed up to take revenge on Epirus for Pyrrhus campaigns in Italy, unnoficially they just wanted an excuse to have a foothole in Greece to more easily take over the area.

While having them as a full faction would probably be a bit broken here and there, although possible, having them spawn like the Mongols do in the base game once someone becomes the Hegemon over Greece like a "End Game Crisis" like a nice send-off in the Campaign.

Can be expanded to work like the Katzen Imperium crisis where you can interact with them so maybe you can "Semi-vassalize yourself to them" so they will help you out but slowly take over your country, until they straight up demand annexation and if you say no they break off diplomatic missions and 100% declare war on you.

Idk, just a thought while the mod downloads for me.
(If it turns out something like this is already in the mod I'm gonna look ridiculous).