I must say, i also had this in mind.
I dont say, comepletely forbid diffrent cultures from marrying each other.
But in 2nd generation aserai and the other non imperial Kingdoms have 80% empire leaders, just because there are so many empire characters.
I rarely find battanians anymore and the khuzaits seem to be extinct.
There could just be a negative modificator for diffrent culture in the progress of the AI choosing a spouse.