IIRC, Doomdark has said that culture will only pertain to characters, not fiefs, so it will be interesting how it works out. That said, I would like to see some events:
(1) merging cultures together in a dynamic way: e.g., Norman father + Armenian mother = something new
(2) religious events that deal with the slow process of religious syncretism along Christendom's borders: persistence of folk or popular beliefs (as a continuity between the old faith and the new) or minority faiths (resistance to conquerer's faith), religious co-existence, etc. There is a lot of room for representing Christianity's diversity, as well as that of the pagan religions, Islam, and Judaism. The various crusader states in particular had "oriental influences." I am thinking of Portugal and Castilla in particular, but also the Kingdom of Jerusalem, and we can imagine if more of North Africa had been conquered and held by the Western Christians (e.g., Sicily, Iberia, Occitans) some interesting cultural things might have taken place.
There should be policy decisions for inclusiveness of conquered populations or maintaining the superiority of the conquerors. I don't know if this is the typical experience, but in CK1 I tended to allow Muslim rulers to keep their titles (sheiks and beys at least) and grant ducal titles to my sons or to archbishops, and this resulted in a diverse population that within a few generations were nominally Catholic.
