The succession laws in the Russian principalities are currently exclusively agnatic-cognatic gavelkind. I don't think this is a very good representation of the Rota system. Admittedly some historians doubt the existence of this system, but historians have also doubted the existence of the feudal system as a truly representative social structure too (I'm inclined to agree with them, but this is a game about feudalism so that's fine). So I claim innocence by association.
Anyway. I think the Russian principalities should have their own unique succession laws. It'd be a sort of mix of gavelkind and seniority. The idea is that brothers inherit before sons, and after that the sons of the eldest brother. Senior heir gets the primary title, younger heirs get other duchies. Only family members with a Duke-level title or the children of Dukes can inherit (this might be hard to implement). It's a lateral model rather than the current form of gavelkind which is linear. Furthermore, the principalities should have agnatic law rather than agnatic-cognatic.
This is a rather specific suggestion, but utlimately what I'd like to see is Paradox giving Russia some love