Some Explaining

One of the very biggest delights back upon my introduction into the OSR is just how flexible the core B/X ruleset is at emulating other genres, generally speaking it's pretty good, not perfect but quite flexible, and it comes with the general added benefit of it being easy to convert content from other such derived systems.

As you well know, but one system I want to get into but it actually looks like a hassle to play is Ars Magica.

I'm much aware of Atlas Games owner being a bit of a TDS sufferer, however from the core book of Ars Magica, and some of the supplement books I can't really see this having tainted any aspect of their work, at least so far as I've surveyed.

The thing which most caught me about Ars Magica was how rich the magic system was, it strikes the right balance between freeform, DM fiat, and structure, with good level guidelines and understandable spell formats that are easy to read.

Thankfully for my purposes and also due to archivists you can practically play Ars Magica in an OSR game right now due to this handy d20 conversion.

https://web.archive.org/web/20090603103000/http://www.users.qwest.net/~jordanerik/Gaming/ArM/index.htm

For something like Lion & Dragon, Wolves of God, or Middle ages Historic centered OSR games this would be perfect as Ars Magic (while not strictly a real hermetic practice) does in fact reflect  medieval cosmology and world view.

As a side note, I'd probably use the Ars Magica to Gurps conversion rules for the straight spell casting functions as the formula's presented above is a bit overdone, or simplify the casting roll to d20 roll under, the DC set at your total of Technique and Form subtracted by the spell level.

What is the point of this post.

Generally the system is functional, but the question lingering in my mind is how one might do up a magic system like this as more setting agnostic? if you wanted to do a more traditional Gygaxian setting or Gonzo D&D game instead of a medieval one.

I've considered using the spell schools from AD&D, but that doesn't really click for me, so I'm currently at a mental stump in the road.

You could just run Ars Magica in a different setting.  You could eliminate the Houses of Hermes if you want, and if you wanted non-PC spellcasters to be potent, you could use the Mythic character rules to make potent non-mages.  And you could even make clerics with the Ars Magica system (there's a sourcebook that has the Major Flaw: Conditional Gift- the condition being you do what your god tells you to do).   So Ars Magica in a D&D type setting is easy, but mages will be much more powerful than non-mages....

...but aren't they always?

Haven't played AM, but I own and have read the 4E book.  To me the magic system looks very setting agnostic.  The problem with using it with D&D is that AM is skill based and D&D is class based.  I don't think there's a good way to put AM in D&D and have it work well without it having it's own unique character advancement, which sounds like a pretty awful game.  If I wanted AM magic with D&D feel, I'd probably port the AM magic system into my favorite skill based game, then translate the D&D spells into AM type spells, frex, fireball would be a Creo Ignem level 25 spell.

They basically have all the classic D&D spells in the main rulebook already (Web, Fireball, Entangle, Hold Person, Invisiblity, Charm Person, etc)  Just under different names "Ball of Abyssal Flame" is Fireball, etc.