I'd like to see a greater focus on peacetime activities and the actual aspects of rulership as well as more focus on dynasty management. Cadet branches have long been a thing that should be in a dynasty simulator game that seem notably lacking in this game. Same with bastard dynastys like FitzRoyal.
Really though, internal focus on rulership would be key. Right now, the game is a lot of waiting around for the next war, but I don't really feel like a king or whatever running an actual country. I might have the odd vassal rebellion or the odd uprising but most are put down pretty quickly. I want to have event chains detailing some of the actual decisions rulers were forced to deal with that could have a positive/negative influence on your vassals because as it stands right now I feel like the only interaction with them is negative or thinly positive, i.e. giving them titles or putting down their rebellion, so something that makes that actual task of rulership feel more weighty and interesting.
I'd also like more events around dynasty management. This includes a branch that is too attenuated being ordered by the pater familias, you, to stop using the family surname. Or dealing with your nephew hauling off and marrying a commoner and bringing shame to the family name. I recently had a game where my king's brother, and the king's heir, married his only son matrilineally to a random countess. How did the head of the family not put the kibosh on that?
I just generally want more stuff like that. After all, this is supposed to be a dynasty simulator at it's core, so I want the mechanics we have to be deepened, not have them throw some other random system in there that will need another couple DLCs to be kinda fleshed out.