ok so hear me out. Christian-Hellenic syncretiztaion. You must control all 10 holy sites have 10000 cash and 3000 piety and it creates a new faith that gives every ruler a choice to convert to. It may be a little overpowered though.
Right, I didn't even think of this. I guess that would be close enough to OP's idea.Unless someone seriously changes something, Reformed Hellenics already get the synchronisation reform as part of their unique "civilised" reform in HF, allowing them to (by decision) get tolerance of Christians.