To preface this, I think the culture system in EUIV is a sadly underdeveloped facet of the game that could really use some extra love in future updates. The issue I want to bring up in this thread, however, is far simpler to solve in the upcoming 1.5 patch.
When I first got EUIV, I was expecting the Basque culture to be included along with the other Iberian cultures in the same "Iberian" group. After all, they had been grouped with the Castilians, Catalans, and Portuguese in both CK2 and Vic2, so my expectations had some precedent to them. When I saw that the Basques had their own culture group (of which they are the only member) and are NOT an accepted culture even for Spain, I was left very confused.
Now, in this scenario I see many similarities between the Basques and Hungarians. Both cultures are very ethnically distinct in terms of language and genealogy in comparison to their neighbors. I can practically hear the previous kerfuffles of "But the Magyars are Uralic! Why aren't they in a unique culture group?" However, EUIV has the Hungarians placed in the 'West Slavic' group while the Basques have to contend with complete cultural isolation.
Paradox's logic behind this is that it is not worthwhile to make a whole new ethnic group just for the Magyars, and they shared many similarities with their Slavic neighbors in terms of culture and common rulers. Yet why isn't this same logic applied to the Basques?
While the Basques spoke and still do speak their own language, they were very close culturally to their Iberian neighbors. There existed no deep animosity between Basques and the Iberians in the EUIV period, which the 'Not Accepted' modifier seems to represent. Furthermore, for the vast majority of the EUIV era the Basques lived under the rule of either the Spanish or French crowns.
The situation grows even more confusing when one looks at the Basques' "cultural unions." The Basques can form both Spain and France. However, they are not an accepted culture in either if another country forms them because of their cultural grouping! While this is realistic in the case of France (the Gascon culture is basically Francophone Basques), it is absurd in the case of Spain.
Looking back at the Basque's history under Spain, one can see many examples where the Basques were actually better off than the Castilians themselves! The Fueros system gave the Basques nearly-unprecedented autonomy for the EUIV era. They had their own legal codes which the Spanish King was sworn to uphold, they had legal protection from torture, were exempt from conscription, and had their own local parliaments.
This, I believe, fully qualifies the Basques for being 'Accepted' in Spain under EUIV's definition. Since Spain operates as a cultural union, I think this provides even greater impetus for scrapping the 'Basque' culture group and fully integrating them into the 'Iberian' group.