In general, the mod adds all East Slavic cultures that are present in the game to the world map. Replaces backgrounds for all Slavic tribes with Norse ones, which looks more natural to me. (only for tribal age)

What does this Mod overwrite?

Sounds fair enough to me.

Could also add some more names to the name_list as I have learned there's so many slavic names that are not used in the game, and instead they put in foreign names. Not counting german names as that makes sense, since slavs and germans lived as neighbours for centuries

My knowledge on south slavs is lacking, but with west slavs, mostly the "Polish" culture in 867 existing before those tribes being unified is odd, just like you felt about it all being "Russian".

I strongly believe Polish should only be a thing in 1066, while in 867 it should be split up into "Polan/Polanian", "Silesian", "Masovian", "Vistulanian", "Lendian", and maybe "Kuyavian" but not sure about the last one, as i'm not sure if the Kujawy region was independent enough.

Also what bothers me about 867 Slavic cultures are the christian names, again, it makes sense in 1066 which is already 100 years after Baptism of Poland for example, but in 867, Christians are a hostile faith to the "pagan" slavs, so it would be weird to use their names. Its just immersion breaking imo. i tried editing the name_list file, but like mentioned before, I'm an amateur when it comes to modding.

What fixes would you like to see for West Slavic and South Slavic cultures?

Backgrounds have been fixed for all Slavic tribes (not just the eastern group) and also for the Slavic religion.
Meanwhile, the culture of the Western and Southern Slavs is fully represented on the world map in the vanilla game. Therefore, there is no particular need to fix it somehow.
However, Eastern Slavs are presented on the world map only as a single culture, that is, super simplified and this has been fixed.

Great mods (this and Slovianska Pravda - Expansion), while you're at it, it would be appreciated if you could also fix up West Slavic and South Slavic cultures.

PS. I have been waiting for slavs to be fixed up, i would do it myself if i only knew how to mod