About CK 2, I hate that ALL PAGANS are Prestige based. I understand this being a thing for Norse, because they can do things that give prestige easily. But for others? Other pagans don't have all those features for prestige. I liked that Paradox gave Nomads their own "thing" so that they would not just be Steppe Norse in spirit. They should do that with other tribals aswell.
Examples. These probably don't reflect reality at all. These are just examples of what might be possible.
Slavic - money based. However they don't get their money from their holdings. Their own holdings produce almost a little money as typical tribal norse holdings. Instead, to produce money, they must build trade enclaves in neibghouring contries. These trade enclaves would be the lesser variants of Merchant Republic Trade Posts and also lesser variants of trade posts on the silk road. Slavic rulers would have access to similar CB's that Merchant Republics have. Embargo and seize tribal holding against valid targets. After all, that is exactly how slavs ended up in the area that is Rus/Ilmen. And due to their more money focused gameplay, their realms would convert to land based merchant republics majority of the time. (This would also imply that gameplay for land based merchant republics would be included.) To reform their religion, they would need to build a certain amount of temple holdings to get enough moral authority.
Suomenusko - piety based. Suomenusko on the map extends gigantic swathes of land. And within that area, everyone practiced their faith differently. So, Suomenusko Religion should be completely composed of heretic religions. This would also require there to be a feature where Suomenusko has no non-heretic religions. And each heretic-religion should encompass an area in size between a regular duchy and a regular kingdom. They would all function the same, but would just be heretics to each other. Most things should cost piety instead of prestige and money. Their levies should be even smaller than they are currently. But they should be able to join defensive wars against other religions no matter the CB used. They also should be able to create Holy Orders in a similar way like Nomads can create mercenaries. To reform the religion, their goal is to convert all(or enough) heretics to one heretic religion. (They DID all have a very similar religion, but they all had their own version of it, so you need to make one version the de fact version) And they should most of the time convert from tribe to a theocracy. They are not Christianity, thus they don't really have to have celibate leaders controlling the theocracy. Their theocracy would for example work kinda like a hybrid of a Merchant Republic and Elective Monarcy where the most pious rulers get elected.
Romuva - moral authority based. Due to this religion's small size, this religion should not go through a reformation. It should be reformed from the start because its religious cohesion should be pretty good. However, its Moral Authority should be abysmally low. Perhaps put the Holy Sites more further apart or make Holy Sites give less Moral Authority. But in return, Romuva should have more means to raise its Moral Authority. They should get like 5% Moral Authority from holding another religion's Holy Site. They should get 1% Moral Authority for each Count level character of Romuva religion. +5% Moral Authority for each Duke level character of Romuva Religion. +10% Moral Authority for each King level character of Romuva Religion. +20% Moral Authority for each Emperor level character of Romuva Religion.
I will say it again. These are just examples to show possibilities and may not be entirely accurate. However vanilla CK 2 design has always been about finding a balance between "fun gameplay" and "historical realism".