For the record, I don't want to play as Pagans, Republics or Theocracies. The game couldn't handle them well enough IMO, and it wouldn't make any difference to the game if you don't play as them.
On the other hand a DLC that added lots of interactions with them, now that I could get on board with. Things like hiring merchant fleets, arranging a coup d'etat in a neighbouring republic, sending missionaries into pagan territory, having your younger son join the church, and later he becomes Pope - the sort of thing that makes a neighbouring republic feel like a republic, not just another feudal state with different inheritance laws.
I think some things (especially College of Cardinals) are surely inevitable, I just hope Paradox doesn't fall into the trap of letting you play as, say, a republic, because I don't see how that could be a a particularly fulfilling or well-fleshed-out role...