In watk3 I divided French into French, Occitan & Arpitan. Atm I'm thinking about a setup with 4 cultures: Francien, Norman, Occitan & Arpitan, with French possibly emerging as a unified culture later in the game. I'd like to see a similar later unification for a number of other cultures (most notably German, Italian, British, Russian, possibly a number of other as well).
Slavonic was divided in watk3 into slavonic & macedonian (for bulgaria and such). Dividing macedonian further wouldn't make much sense imo. Dividing Slavonic is somewhat problematic as well as it would yield really small cultures (assuming that you would suggest dividing it into, say, croatian, slovenian, serbian & bosnian which seems the most logical to me).
Baltic I did not consider yet - it currently merges Prussian and Latvian region. Splitting those will start to make sense if I include that Kleinlitauen province after all I guess

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Regardig other EUropean cultures (as I assume no one is interested in my fantastic American setup

) I for now handled the following splitups:
Scandinavian -> Norse, Danish & Swedish (like in watk3)
German -> Markish, Low saxon, Upper Saxon, Franconian, Swabian, Bavarian, Austrian, Silesian & Teutonic (the last is the only addition compared to watk3)
Anglosaxon -> English & Scottish (alike in watk3)
Gaelic -> Irish, Gaelic (for Scotts Gaeilic) & Brythonic (for Bretagne, Cornwall & Wales)
Iberian -> Castillian, Catalan & Portuguese (alike in watk3). I also split Andalusian off of Berber for Iberia.
Swiss remains a nice, not-linguistic culture.
Dutch -> Dutch & Flemmish, though alike in watk3, Flemmish corresponds with all of the Southern Netherlands rather than just the DUtch-speaking one's. Not sure if it is sensible to add Wallonian.
Russian/Ruthenian: Initially I thought about simply reliving Ukrainian and call it a day here. However, based on 15th and 16th century situation one could also reassign those into 5 or 6 more local cultures that could eventually merge and/or divide into Russian/Ruthenian/Ukrainian in the 17th/18th centuries. I consider something in the line of Pskovo-Novgorodian, Moscovian, Volyhno-Galichan, Turovo-Kievan & Minsko-Smolensko-Ryazanian, the last probably split up further into west and east branch. This isn't however a very useful naming so I'm undecided on this yet

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Ugric -> Finnic for Finland & Estonia, Permic for Volgaic territories & Mansi or Yugra for Ob-region.
Other European cultures I haven't got to as of yet (I split up cultures as Arabic & Altai as well but that's already Asia/Africa)