@petertel123 Well, this is how culture defined in ethnolinguistic and anthropology:
"As i see it, society's culture consists of whatever it is one has to know or believe in order to operate in a manner acceptable to its members ... Culture, being what people have to learn as distinct from their biological heritage, must consist of the end-product of learning: knowledge in the widest sense of the term." (Goodenough 1957, cited in Cerri-Long 1999, p. 88)
^ This is being a quote of a quote from a book on a bit more broader matter.
Still, I don't argue with you on this one.
You could also check Wikipedia (though, not being the best resource of knowledge, but at the very least you can check it anytime) for its define of ethnic group:
"An
ethnic group or
ethnicity is a category of people who identify with each other based on common ancestral, social, cultural or national experience. Unlike most other social groups, ethnicity is primarily an inherited status. Membership of an ethnic group tends to be defined by a shared cultural heritage, ancestry, origin myth, history, homeland, language and/or dialect, symbolic systems such as religion, mythology and ritual, cuisine, dressing style, art, and physical appearance."
You may argue that ethnicity and culture are different things and you will be right, but here is what I'm trying to say:
Novgorodian, Muscovites, Ryazanian (and any other group you may bring to the region in EU4) would be in a Russian ethnic group.
But what makes their cultures different? The fact, that Novgorod was a member of a Hanseatic League? No. The fact, that Novgorod was a sort of a Noble Republic with its Veche. That sounds better. But you shouldn't forget, that those Veches weren't common only to Novgorod. They were widespread through Russian (and Ruthenian) lands. Though, under invasion and rule of Mongol Empire (and its successors) the tradition started to fade because of increasing power of Knyaz. In XIV century Veche was pretty much abolished in Muscovy, yet it was intact in Pskovian Republic, Novgorod, and, iirc, Vyatka. There were local veches in Ryazanian, Muscovian and other princedomes, but they were something on a level of villagers getting together to decide what do with crops (as a rough example).
Though ethnolinguistic does make a strong point of language difference as a way to differentiate between cultures, dialects of the same language aren't enough on their own to split cultures.
So, what we've got? We've different government types and dialects. Here we can add that Novgorod and Muscovy were rivals.
Still, though many consider that Novgorod didn't take a part in Battle of Kulikovo, there are mentions of 7000 Novgorodian warriors sent to Great Knyaz Dmitry Ivanovich. Furthermore, Novgorod was Orthodox (as all other Russian princedomes), it was a part of the so-called Kievan Rus (though, nowadays here in Russia, it is called "Древнерусское государство" (Drevnerusskoe Gosudarstvo). I don't want to start another "holy war", so I won't make any arguments about both naming. But I would notice, that Novgorodian land is considered as the one "where it all started".
Furthermore, the way people lived in towns and villages of Russian princedomes were extremely similar. Saints and Old Heroes were all the same.
As you can see, this is a rather hard topic. You can argue for Novgorodian culture being something completely different from Muscovite (or even Russian), and you may argue that they are just the shades of the macro-Russian culture. While I don't like (as I already said before) when Muscovites and Novgrodians are divided into different cultures (especially when it effectively destroys Russian culture as a thing in the game), I still believe, that if you started to diversify it, you should do it to the end. There should be Smolenskian culture, as a vital part of Russian ethnos. There should be Mordvins (as a half-mordvin I want to see them in-game). Ruthenian lands should be split between different cultures within one ethnic group. So shall be made with Polish lands. With Ottomans lands. And, may be, with Japanese lands as well.