You could also try bulgaria in 867, some years after the beginning of the game an event will fire: Bogomil begin to spread his heresy in the kingdom.
Bogomilism is in CK2+ a very interesting heresy, because they have many malus at the beginning, they have their own government and you will have to take many "reform" of the faith and it will become a very strong christian doctrine. But you will have many foes, especially a certain Emperor...
Personally, I tried bogomilism with Ivaylo Bardovka, the peasant "emperor" of Bulgaria from 1278 to 1279, to go more in the destruction of the "old regime"