I don't agree about your example of Andalusian culture. In real history, it was as you say a melting pot of those three cultures. But in-game, it's a subculture from the arab group. So, for people of iberian culture, andalusian are as foreigners as any other arabs (maghreb, etc). So, when (silly me) i moved once my capital to Toledo before it turned castilian, and my first born son turned out to be of andalusian culture, he had a big -18 to all relations with his northern vassals.so some "cultures" in-game are already sort of melting pots already, if you know the history. Andalusian, for instance, can easily be considered a melting pot of Berber, Iberian, and Arabic; Saxon is actually Saxon + Norse (due to the heavy Norse influence in England for the previous few centuries); Turkish in Anatolia is Greek + Turkish; etc.
There could be perhaps a new culture andalusian_iberic, I don't know.
And for the other melting pots, not the historic ones, but the ones that could happen, I will be more than happy to see them.