Hermetic magic is a fusion between ritualistic magic and spontanious european magic from time before cult of mercury and rome took over.

Following Diedne magic virtues guidelines, non hermetic magic is good at spontanious magic, but perhaps not so good with formulaic magic.

Good, it fits perfectly with Dark Ages: Mage (or damage among friends)

European wizards use simple spontanious magic system:
4 forms, no techniques. Casting spell is form + sta + die (+ bonuses)

You match desired level, no fatique, you fall short under 10p, you pay with fatique, fall short by 10 levels, you fail.

Witches (Verbena) have 4 forms:
Winter, spring, summer and autumn.

Opus dei, catholic magic:
4 arch-angels
Michael, raphael, uriel and blahblah.

Vikings (Valdaerman) have also 4 forms:
Wiseman, soldier, what ever, I do not remember.

these traditions do not have hermetic flaws, so they usually do not need vis for permanent effects - healing is a normal spell that works without vis, with permanent duration.

Dark ages Fae adds a lot of interesting ideas also.
Faeries have 4 Dominions, 1 rating turns to lvl 10 in ars magica.
All that is said about faeries in that book fits seamesly (in my opinion) with Ars magica. I should write a conversion guide someday.
Dominion
Day: Power over light, faith and thought

Dawn: power over senses, body and time
(healing, shielding)

Dusk: Power over spirits, magical travels and gateways
(reading minds, controlling air, viewing past events)

Night: power over darkness, nightmares, cold and misery

simple thing is to take a form of animal, rank 2 or 3 (20 or 30 level spell) so it is natural for faeries to alter their shape as it is made of dreams anyway.

Sounds possible, but you'd need to write up a slightly more detailed "guideline" for each Pillar.

I'd also throw in limits for each tradition, otherwise they are all more powerful than Hermetic magic, which does have limits (such as needing vis, etc).

I do not think so, 4 forms against 10 (well, more limited) forms and 5 techniques.

Tech + form combination gives higher results so while their theory would be more versatile, it's the power that counts, and hermetic formulaic magic is more powerful, especially when you add parma magica.