In a tangential question, what kind of market does anyone expect there would be for an alternate history supplement?
My concept is this: Ars Magica has two conceits in conflict (for the purpose of this supplement) 1) that the order's existence, and magic in general did nothing (substantial at least) to change history and 2) that Bonisagus was in correspondence with Charlemagne.
However historically Charlemagne enslaved all (proclaimed) wizards to the throne, except those who were diabolists or sacrificed to pagan gods, whom he executed.
So what if Bonisagus' letters to Charlemagne changed that history, and thereby changed the history of what would, in the prime timeline become the Holy Roman Empire?