In Decline and Fall of the Roman Empire there are two little pieces I tthink will be very useful for Ars Magica.
The first is this: some ancient Romans apparently had cloaks made of the beards of shellfish. Ars has always lacked an Aquam founder, although one pre-Hermetic maga who was a water specialist was described in Sanctuary of Ice.
Second: Gibbon insists Mongols are monotheists. This means that, in Ars, they have Wrathful Dominion backing their invasion. Of course, that makes God kind of a jerk…but he did back the crusaders and jihadists, didn’t He?