That's going to be great. The Pope could be invaded to depose an antipope.
Sadly too few Catholics left these days. It's basically just a few duchies worth in Europe and one of the Eastern European Kingdoms no one usually cares about (it's not Pomerania or Russia, but it's in the general area) now. Everywhere that used to be Catholic is Sunni, Shiite or Germanic now. Orthodox Christians are the only branch doing ok since the Sunni Caliphate wrecked the Miaphysites.
Luckily for me the unkillable Shiite army of 31000 event troops finally died not long ago. Decadence revolt of 60k overthrew the Caliph and all the event troops despawned and the Caliphate exploded into chunks. Not been able to take advantage because of a combination of threat and my attempts to form the HRE destabilizing my own realm. Sunni Caliphate exploded not long after.
Just me and the Byzntines left as far as big guns go, at least till the Mongols and the Aztecs show up. Black Death showed up a long time ago, killing remarkably few of my family. Lost a wife and 2 kids out of 6 to it, cured myself with the powers of Hel. Want to take over the Byzantines, but getting a strong claim is proving difficult, might not be able to pull it off unless I go Christian forever.
I might try an Ironman run in India, or see how well the secret religion stuff works as a Zoroastrian under the Sunni Caliphate.
Just had a thought on this topic. Could you potentially do this if you use something along the lines of the 'Create a Kingdom' event?
Maybe? Not sure if a generic religion template is a thing in this game, I think they generally need to be defined properly before the game starts.