I am just ruminating on some thoughts here, so feel free to ignore or castigate as you please.
Basically we know that in the 2nd Century BC a wizard priest of Mercury came up with a magic spell (the precursor to what Hermetics call "Wizards Communion") that allowed assorted Wizard-Priests of other Roman cults co-operate in casting their Great Ritual magics.
38 different Great Ritual spells, so presumably 38 different priests. From presumably 38 different temples. But were they all different cults, or did cults have more than 1 temple?
How many cults were there anyway? 2nd century BCE means the Punic Wars had happened if I recall my history. So Rome was expanding out of mainland Italy.
So as well as native Cults such as Jupiter and Quirinus, I presume the Greek influence had been established, and Greco-Roman cults were about, Maybe some more exotic ones like Greco-Egyptian - Ammon perhaps?
But Mithraism? Possibly too soon.
So, lets us say that Plentarch took a bunch of Mystery Cults, and managed to weld them into a Tradition. A Tradition that could later accept foreign cults adopted by the Roman Empire. Such as Mithra.
But what else?
After the Republic fell and Empire arose, there were cults dedicated to former emperors. According to HoH:TL, Guernicus was the last survivor of Terrae wizards, yet in the Wikipedia article, Terra is referred to as an abstract deity.
Alan Moore claims to be a follower of Glycon, a "religion" introduced in the 2nd Century AD, by one of the many "prophets" out there. According to the Wikipedia it lasted for at least a century, and a line from Horace suggests it predated the prophet. Became popular enough to feature on some imperial coinage.
Could this have been instigated by a Mercurian priest-wizard who outgrew his own temple, or was integrated into the Mercurian Tradition?
After the fall of the original Cult of Mercury, was there a lineage of wizards from the priests of Glycon? Or the priests of Imperial cults?
We know there was lineages from cults of the underworld (Guorna -> Tytalus and Tremere), Mithra (Flambeau), Meriniata might have been descended form the Cult of Diana, and there is mentions of the Cult of Orpheus, and the Cult of Vesta, et al.
Where did the priests get their Great Rituals from - the gods they served?
Why would the wizard-priests have to cast the spells, rather than the "god"? I am thinking of one of the novels by Diana Wynne Jones, in the Chrestomanci series, where a child enchantress is enthroned as "the living Asheth", and all the miracles performed in the name of 'Asheth' are actually magic unknowingly performed by the enthroned avatar, as the diety is normally too lazy to do it herself.