Hello, Everyone !
(edit on 25 october, 2024: replaced Major/ Minor Domain by Greater/ Greater Domain, just because Arm5 already has a lot of Major/ Minor. It also makes abbreviations easier)
I originally planned to submit this to Sub Rosa, but life got in the way and… Let’s just say I’m not the gamer I once was, and this is proving difficult ( and slow) to write. So, please, bear with me.
So, without further ado, I’ll just go & write. I’ve been thinking about the Origins of Magic, and what kind of Sorcery might have been practiced by the Founders I had other ideas, but they involved gods, and Angels)
The idea was, we have a rough idea of what kind of focus they had, like Fire for Flambeau, Ghosts for Guorna (Probably with a side note of Corporeal Necromancy), Stone for Guernicus… And we know that, working with them, Bonisagus invented the 15 Arts we know and love. So, there must have been some similarities between both their Magic and the Hermetic Arts.
Note the key word : Focus.
Cutting short ( And, I’ll admit, typing with 1 hand, my middle finger is beginning to ache due to the repeated strokes 
), I figure they had what I call Domains, both Major and Minor, either fast or slow. A Greater Domain is about as wide as a MMF, whereas a Lesser Domain has a width comparable to a mMF. Fast means they progress as an Art, Slow means they advance as Abilities. In my head cannon, they each had 1 fast GD, upgraded, with a bunch of slow LD
If we add the Platonic Forms, between all the Founder , we probably can see the outline of the Arts. Now, Bonisagus just has to observe that, Some times, both Flambeau and Guernicus created Fire or stone, elements which they also could control…
We also know Hermetic Magic has its roots in Mercurian Magic, which was big on Rituals
Unless i'm wrong, we also know that Spell Mastery has its roots in that pre-hermetic magic, so, that was probably a big component.
You get the idea ?
Now, the next (or rather, Previous) part : the Parma. IMO, Boni ( who, IMO,must have had a Major Domain in Dispelling, or something like that) used OR, experimenting with… What kind of spells ? Probably something like « Impede the Intermitent Inteloper » (TME p 109), but based on the more general"Dispel any magical effect" rather than"Dispel a specific type of enchantment" guidelines. Let’s Call it "Stop dead the hostile magics" (PeVi Gen, R: Per, D: Sun): This spell prevents the target from casting, or being affected by spells of a level less than the (level of this spell + 2 magnitudes+ stress die (no botch)).
From several iterations of that spell (and variants), He got enough Breakthrough points to invent the Parma.
So, what did the Domains cover? the 10 Hermetic forms (that's roughly about 50 possible Minor Domains, or 10 majors), obviously, but also the Amazon Arts (RM adds 15 Minor Domains) and probably also Virgilian Magic (also in RM).
How Did you learn a spell? That's what Spell Mastery is for! 
Let's say you improved your Mastery until (Applicable Domain+ Mastery) was higher than the spell's Magnitude.
How did spellcasting work? I figure you used Sta + twice your highest applicable Domain + Spell Mastery + Aura. This means that, yes, inside their Major Domain, founders ( and similar magi) were probably stronger than modern magi, while being at the same time less powerful outside of it. they compensated by having the first Mastery ability be Ceremonial Casting (here's your Mercurian roots! 
) and using Vis ( up to your Domain, for more Mercurian tie-in).
As an example of a pre-hermetic sorcerer, let's give them Stamina 0, 1 Fast Major Domain in Birds at 15, and 5 slow Minor Domains ( Healing humans, Control Fires, Destroy Demons, read thoughts, and Transform Water) at 3, with AL and Philosophiae at 5 both.
they probably know a bunch of bird-related spells at Mastery 1 or more, for which they have a CT of at least0+ 215 +1= 31. so, they can reliably cast Bird spells up to level 30-35, or 40-45 if they take the time for a little Ceremony 
.
Outside of that, they have to rely on Ceremonial Casting to reach a CT of 0+23+10 + Mastery 1= 17, and their low Domain limits them to spells around the 4th magnitude anyway (level 20). 5 magnitude is possible, but they need more study (Mastery 2+) and it is harder (Ceremonial CT of 2*3+10+2 = 18 or more), probably requiring vis.... or the missing bit! Wizard's Communion! And Names of Power, let's not forget Names of Power, for more heathenry and Pageantry!
So, that's it for today. Sorry if this is too rough and I couldn't write more, but the ball's in the air, I hope you'll have fun with it.
And that bit about inventing the Parma? Developing an ability from a spell? I figure it could be done again with, for example the Pyromancer ability: you can create balls of fire, or bolts of flame, doing +5 damage per level in the ability to anything you can see with a penetration of 0. You may lower your damage by any amount, to increase your Penetration by the same amount, or affect several targets ( -5 damage per target past the first)
Besides the intellectual exercise, I figure this can give a framework for stating time lost magi (Tytalus is probably not the only one lost in a slow time regio) or playing in a "before the OoH" saga. This can be fun, as they'd be both weaker and more powerful than their Hermetic successors, with maybe a bunch of weird spells ( Amazon and Virgiln effects). Probably exactly as they should. 
Thanks for bearing with me 
.
. Likewise, the existence several, Intellego-based, minor Domains seems both reasonnable and credible, and a further lead on a Technique separate from Forms 
)
( yes, there's a Mage smiley! 
. Most notably, his invention of Pilum of Flame.
)
and her pupils
, Merinita 
and Bjornaer 