It might be cool if they could work schisms in somehow. Imagine if after a pope dies you declare one of your own cardinals pope or whatever and other catholic factions have to decide which pope they follow. If you follow a different pope another faction would like you less. You could start your own crusades and the other half of catholicism would be less likely to join them or allow them to pass. As if the province they pass through has half the zeal or something. I don't remember if the Schisms ever caused big wars? It migh complicate things a lot and wouldn't be all that historical but it would be interesting.