I'm sure there would be demand for it.
I guess it depends how difficult modding is, and the key would be how much you can tinker with senate/mission system to turn it into papacy but also add spice for non-catholic factions.
after that turning it medieval shouldn't be outrageously difficult in itself unless CA goes out of their way to cockblock it: Adding new armor models & textures, new maps for medieval castles & cities. It would be a lot of artwork/mapwork, and would depend on the organization and dedication of the modellers and designers in question.