Dynamic Nationalism would be fun, not sure how well it'd be implemented but I'd be curious to see how it was implemented.Something that I think may be interesting, though definitely more of a stretch goal, would be some attempt to model emerging nationalism.
Ex from one culture -> culture group -> geographic group depending on things like economic ties and colonization
ex an independent Sokoto surrounded by colonists may come to represent a Pan-Africanism, but alone probably wouldn't.
Or a strong Balkans with diplomatic ties may lead to a Balkan federation
this could also go in reverse with a weak Italy perhaps separating into a south Italian vs North Italian nationalism.
To be fair, there are still Zoroastrians to this day, so there SHOULD be Zoroastrian pops somewhere or another.And don't forget the Achaemenid Empire!
*You must start as a rebel released one province minor inside Persia with the one zoroastrian pop causing the rebel uprising