I (we) created a fantasy world which was a bit low fantasy at the centre regions of the game. I was introduced some bits of Game of Thrones elements and but it mostly looks like some low renesaince/high medieval but it has elements from dark ages or early medieval era.
There is only one 'real' realm which is magic. It is a bit like Chaos in WFRP so everyone uses it. There is another kind of realm which comes from the people's belief. This is a kind of antiMagic realm. It is a kind of Dominium aura but generally doesnt give more penalty to magic than a level 2-3 dominium in AM.
Priests use magic realm but they are kind of attuned to this anti magic realm so they dont get the penalty. They tend to use divine or infernal powers depending on the patron god. Mages are in an arcane order similar like Order of Hermes but they tend to get their initiation from a special space beyond time and space.
Limits of magic are not a problem for us.
You can easily add or remove to this. It all depends the group and their flexibility
I don’t see why you would need to change the physics. Correct me if I’m wrong but the books(especially Art & Acadame) consistently imply that ArM takes place in a world where Plato was just plain right, at least for the most part. On that note a modern world that runs on platonic physics would be pretty interesting.
The physics of Mythic Europe - as described in A&A - are scholastic physics based on Aristotle and hashed out in medieval western and central Europe only decades after 1220.
The base of Hermetic Magic Theory is an eclectic mix of those Platonic and Aristotelian theories available to the most advanced scholars of the later 8th century, and of what Bonisagus could gather on his travels from lost cults and those few magi he could approach (see especially HoH:TL p.4ff The Founder Bonisagus, A&A p.11 The Hermetic Intellectual Landscape and Hermetic Magic and Philosophical Breakthroughs). In particular, adapting Magic Theory to scholastic physics is a job still to be done by the Order of Hermes in 1220.
A world running on scholastic physics is vastly different from ours wrt cosmology, astronomy, mechanics, chemistry and all science and technology depending on it: I reckon that you wouldn't call it 'modern' at all.
The magic system in Ars Magica has been more or less the same since the early editions, long before any of the authors elaborated on medieval philosophy or physics. I don't see much need to worry about Aristotle or Plato in order to use the core system. The few areas where Aristotelian physics are explicit within the system - the sensory rules in particular - are actually the hardest areas to make work within the game rules, in my opinion.
This is not to say that the elaboration on medieval thought doesn't add immensely to the richness of Medieval Europe. It does and that's one of my main points of interest in the game. But there's no barrier to putting this aside and moving to a different fantasy world.
Several magic systems for more generic games have appeared over the years, which borrow Techniques, Forms, spell parameters, defenses etc. from Ars Magica. The last I found was from WOIN - but I don't exactly search for such clones. They are also not, what Jonathan Link asked about.
It is not clear to me what you want to uncouple. The magic system or the Hermetic world view? If you need to adapt ArM to an existing world, you have any number of challenges to get the right feel of the magic for that world. If you turn around, and adapt a world to ArM magic, you probably have a simpler task. And you can always amend ArM too.
The example mentioned with the transition from Platonian to Newtonian word views is a good one. The magic in a world is tightly coupled with its mythos and prevailing explanatory models. If you change fundamental mythical principles, the magic system has to change, not necessarily the mechanics, but the categories and limitations. However, many different worlds can support the same mythical foundations. We have already seen Europe move from one to another. Thus, we cannot really uncouple without a plan for recoupling.
It is worth having a look at /World Tree/ (apparently, I am not allowed to include a link here).
It uses the flexible arts-based spontaneous and formulaic magic of ArM, but most of the mechanics is different, and the world is the most alien RPG world I have ever seen. Notably, it has forms for time and space (in addition to ten forms pretty well matching Bonisagi). It is a good example of how to recreate the flexible magic that I love in ArM, in a completely different setting. I'd love to play World Tree again.