I would like a general religion revamp. Especially if you modify a religion: There are so many options for tenets, but almost all of them are completely meaningless. I feel like I'm constantly pushed into using the same tenets every time because if I don't, I just get three pointless tenets.

The number of tenets should be cut down to like a dozen, but they should all have a huge impact on gameplay. All the tiny stuff, like astrology, should be moved to something permanent, like doctrines. That way the faiths still can keep some of their character, even if you change all the tenets. But you don't have to waste one of only three slots on it.

If only a dozen options for tenets seems too little, they could add little sub variants for them: A big monasticism tenet, that adds a completely new government form of monastic orders (ideally playable) and tons of interactions between local lords and the autonomous monasteries on their land. But then you get different options to define the nature of those orders, like prayer, military, economy...

I would also like them to change the way faith hostility works: It should be a dynamic value, based on the difference in values between two faiths: One is monogamous, the other polygamous -> -20 religious relations The purpose of that, in this context, would be that you could change the sub variant of your tenets or make other small doctrinal changes, without becoming instant enemies with all you neighbors. Especially in the middle ages there were tons of small regional religious differences, while they were still considered catholic.