I have a few questions/suggestions about this mod:
First, wouldn't gavelkind make more sense than primogeniture? I don't remember any instance in the books where any person held more than one castle or even kingdom(paramount) which happens quite a bit in the game. For instance, in my current game, the Iron Islands, Riverlands, and Westerlands are all held by one AI.
Second, is there any way to prevent kingdoms(paramounts) from breaking up into irregular bits all over the map. My OCD causes me physical pain when I see a bit of the Reach or Vale in the Westerlands, etc. Once again, in the books, it's been 8,000+ years of more or less defined borders if I recall correctly.
Here are just some of my suggestions from me:
Have varying degrees of crown law but all crown laws keep vassal inheritance within the realm. Also, I'm not sure how to do it, but I think limiting to only Gavelkind would keep things more along canon lines in my opinion so kingdoms, castles, and titles don't end up bunching up to one person. Perhaps an event could be added for when you inherit two kingdoms/lordships. For example, pretend i'm LP of the Westerlands and inherit the Reach. I get to choose to keep one. The other will be filled by either a relative, courtier, etc. If they are a relative, this could create a cadet branch of your dynasty with a permanent opinion boost between the two families.
Overall, this is a completely fantastic mod. I just feel like a few of the things I mentioned could be tweaked a bit in order to follow the books a little more closely. I'd like to hear everyone else's thoughts on this. I'm by no means a programmer so I don't really know how realistic my ideas are.