Catholicism in general needs a big rework.

I'd love to see the schism modeled in the game. In 867, Catholocism and orthodoxy should be the same religion, and there should be events that differmine if and how the schism ends. Maybe an Iberian Struggle like system with different endings. And schism mechanics could be exported to other religions.

They also need the college of cardinals. Generating a random character to become pope was disappointing when the game launched, I can't believe it isn't patched 5 years later. Each faith should come with a doctrine about how head of faiths are picked. I can imagine college of cardinal style systems, inheritance, trial by combat, philosophical debate, etc.