Will it be linked to Moral Authority (or an equivalent mechanic) or will there be a different system? I'm hoping the latter as the former used to limit playing alternative history with faith in CK2. A lot of religions were no go areas because it was near impossible to convert your counties to them. For example, playing a Monophysite England or an Italian Manichean as both those faiths have Holy Sites almost at the other side of the map and you could have very little impact on the MA of your faith.