Frankly the first time I saw this decision my natural inclination was thinking permanent curia control, but allowing Catholics to still spend Papel Influence on the various abilities. Papal ideas are strong but not top tier and frankly i would rather have Italian ideas. Even with the events for permanent buffs being fairly strong as well, it sort of makes up for the difference in idea set once you bolt on the 10% manpower bonus. Its just that curia controler is just so damn powerful. Also would be intersting if kingdom of god could call crusades AFTER they normally gets disabled.
If there could be one change that would make sense for it in general is that the AE reduction should only apply to catholics IMO.
But objectively from a balance perspective it would probably be one of the best non reduced core cost tags in the game at that point..... you know except form Shan form something else that would result in even more broken fun,if that's still allowed.