Should more mixed cultures be added?
There is English culture which is a mix of Anglo-Saxon with Norman superstrate.
I would like to see an Anglo-Norse culture which would be a mix of Anglo-Saxon and Norse superstrate.
What others could you think of?
Here's southern Italy:
The Sicilian culture could mix with the Arabic and Berber ones, with a Siculo-Arabic culture (basically proto-Maltese) as the result, as well as with the ones grouped into the Byzantine group, giving birth to the Grecanic culture, that would be based on the IRL Griko one. The Lombard remnants in the South, if successful, could result in the rise of a Siculo-Lombard culture inspired by the OTL Gallo-Italic dialects of Basilicata and Sicily. IDK if they kept Albanians in the game or not (they already scrapped Dalmatians for no reason and it's a pity) but the Sicilian culture and the Albanian one would result in the Arbereshe culture gaining a foothold, perhaps even greater than the OTL one. Bosniak, Croatian and Serbian rulers could sponsor a Siculo-Slavic culture modeled after the IRL Molise Croats.
If the mainland and Sicily are separated long enough, a Neapolitan culture might supplant the Sicilian one in the mainland.
Edit: my first CK3 game, as the Orange dynasty of Venaissin, ended in a game over 180 years after the 1066 start date, as the last member of my dynasty died without a valid heir, while holding only one county, that wasn't even the county I started with. The way they reworked inheritance (your culture has to discover how not to gavelkind it all away first) and Tthe county control mechanic helps to prevent blobbing a great deal.
In fact, I spent most of my time improving my demesne, so that even as a two county minor or even an OPM I was rolling in money, comparatively speaking. Playing tall seems to be easier. That said, weird and amusing stuff still happened, such as an Orthodox France that joined the ERE and an Adamite Hungary with the obligatory stark naked king.