I was thinking Orthodox could make more gains. But what would countries like Germany and France use .I have of an Arian Christianity being used in their area
I don't think that would work. The plausibility of such a military campaign aside, what's to stop the top bishop of the most powerful Christian country from declaring themselves pope and carrying on business as usual?
I think Arianism is your best bet-perhaps a Frankish court that remains Arian and encourages neighboring countries to be Arian could marginalize Rome to the point where it becomes obsolete to spiritual life outside of Italy.
Well, it's certainly possible that a more knowledgeable board member than I will have a different opinion, but AFAIK Arianism was the most entrenched 'heresy' in Western Europe before the Catholic Church was able to get to it's full level of control in the pre-Reformation era.