Can you modify it so you get a boost to relations with the Papacy or directly to Papal Influence if the Medici are in power? Or maybe add a "historical friend", though that may be too far... Keep in mind I know nothing about the history here, except what I've read in this thread, I'm just suggesting things based on game mechanics.
I've written an event that dramatically improves relations with the Pope if you become Papal controller. Of course to do that you need a cardinal, but that's up to the player to acquire. The Medici often had good relations with the Papacy because they'd infiltrated the College of Cardinals (and eventually the Papacy itself, several times), which is not an easy thing to do. Not really the sort of thing that should just be handed to the player on a silver plate.