Culture as a whole needs a complete rework. I'm sure we all know the countless errors and mistakes the current iteration of 'Culture' has. (Slovak is apparently closer to Romanian than Czech, Turks are in the same group as Khaleejis but not Azeris, Vietnamese (will be) in the Chinese group, Albanians are Slavs apparently, Beja is not in Cushitic, Finnish is not in the same group as Karelian/Estonian, Ryazanian culture is a thing for a reason, Korean is still the worst culture group in the game, language is supposedly not the standard for culture groups but Sinhalese is in the East Aryan group (but Maldivian isn't?), etc.) Culture needs to have a dynamic cultural similarity meter instead of the fixed groups we have now. Integrating say, Danes as a German nation should be easier than integrating Swahilis.
This would make much more sense. Right now simple diplomatic moves such as military access and fleet basing take up full diplo slots that are equal to a full blown alliance/royal marriage, but for some reason diverting an entire nation's trade to another or a tributary/overlord relation doesn't take any diplomatic management at all?