Personally I have never really liked universal and stable opinion penalty for every culture in ck2. Medieval world was really damn mixed, even if we implement some xenophobia shouldn't it be different depending on the kind of culture? I'd say medieval Polish king should have nearly zero cultural penalty with some of his closest neighbor, some penalty for further European Christians and higher with very exotic Christians and everybody else.
Whatever happens, I just hope different culture isn't as big pain as in ck2, where mechanics discouraged having landed characters, guardians and whoever of a different culture.
What do you think?