Certainly GURPs can model long term advancement, though honestly not as well (it simply uses hours of study to gain experience points rather than having max levels and source qualities of material, but this is easily adapted).
mythic Europe is a fairly simple setting that could also be modeled in a number of systems, including GURPs.
Even the concepts of auras and realms could be ported over relatively easily. what does not add easily is the system of interrelated abilities being able to be added together to accomplish great things, or the design and management of a long term cooperative organization, or frankly a halfway decent simulation of a medieval economy, though I think the last one could unequivicably be improved upon, it certainly would not be worth a new edition to do so.
Finally the idea of different rules sets for different types of characters is something entirely unique to ars magica, and both a feature and flaw- feature in that it allows widely disparate character concepts to be played with rules which emphasize their own perspectives beyond what even a "generic" system can accomplish, flaw because it means new players wanting to pay different character concepts need to essentially learn a new system to do so.
Doesn't an abstraction into hours of study lead to characters working munchkin hours?
It sounds reasonable enough for the occasional one-week downtime, but adding up 52 intensive weeks over a year breaks plausibility. The season-wise abstraction in ArM makes this reasonable balanced. The key point is not the downtime abstraction, but the abstraction being tailored for years of downtime.
Now, I have only scratched the surface of GURPS and that was twenty years ago, so the question is genuine.
Honestly it is a very little used rule, and what it leads to varies considerably based on setting and campaign. in Illuminati University (IOU) in third edition it was used to calculate character points per credit hour per semester based on a presumed number of hours personal study per class hour. in others it has been used to cram study into as little time as possible in order to minimize downtime, or maximize times advancement while convalescing. on the other hand it has bee adapted to regular work schedules to calculate advancement when doing nothing productive besides holding down a job. So it really is an "it depends" situation, like most of GURPs.
As in GURPS is an RP system, it is a system to make RP systems? :- 
GURPS is a roleplaying system, but its utilization is very setting dependant- for example the rules in powers can be applied as a form of magic, for supers characters, or for psi with only slight differences. it also has multiple optional magical systems which can be included in a given game, and rules for technology ranging from pre-civilization to star empires. It stands for Generic Universal Role Playing System.
One thing that could be done to simplify Ars Magica in a new edition, and I know people will disagree with this, is ditch the Abilities (ie the skill system).
That is, rather than listing out a whole bunch of skills and specialties from a big list, simply integrate all of them into the Characteristic scores (as specialities) and/or include particular Abilities as Virtues. So, for a Wizard, as an example, you just have a virtue explaining the type of training they get (in magical theory, Hermes history, etc). If somebody is trained in martial weapons then there would be a Virtue trait to state this, and the Characteristics and Specialities would provide individual variation.
The character generation, in my view gets particularly bogged down in this area - where the levels and distribution of points based on age and experience is rather finickity. A character could be profiled by having 8 Characteristics, each with a speciality which may add an extra bonus, some personality traits, and then a few Virtues & Flaws. Wizards would then have some Arts scores for the magic system.
The core mechanic may need to be changed in order to reconfigure the maths. Maybe introducing 2D10 to produce a bell curve, with a set target number to succeed, and then youâd just add the Characteristic and possibly Personality trait modifiers.
You could do that, and for the purpose of playing magi, it would not be a big loss, since the focus is on the arts.
It would take much of my joy out of playing companions over two decades game time, since we would loose the freedom to choose what abilities to develop. Both grogs and companions would be bound by their starting class, unless I misunderstand your approach completely.
But you are right that the character generation is a chore, and I don't think the change from 4ed was a step forward. Personally, I tend to be sloppy with checking the constraints on abilities. They really don't make much of a difference in the long run, at least if one avoid abusing my sloppiness. YMMV.
If you build the various Abilities into less granular Virtues, it shouldnât mean that Companions are less fun to play especially, but it might mean that Experience becomes handled in a different way. It could mean, for example, that accumulated experience may be able to buy off certain Flaws or gain new Virtues over time.
Right, that sounds more complicated to me, rather than less.
But it one can only truly evaluate the idea when seeing a worked-out design prototype.
A few people mentioned that they don't see themselves switching to a 6th edition because the 5th is so detailed and complete. This made me think over the last few days.
What if the 6th edition were to explore an area that a few saga have, namely the early days of the Order? Set the game in 900 A.D. and that gives you an impetus to introduce a number of different mechanics, with magi that both more diverse and slightly less powerful. In a way, they would be more specialized and closer to hedge traditions their respective Houses originate from. They might not have access to all of the standard Hermetic Arts, or be otherwise more limited in the things their personal magic can do.
From a story standpoint, they would live in a wilder magical world, without necessarily making their opponents more powerful. They would get to enforce the "join or die" clause, or be part of House Diedne.
Many of the things that we now take for granted in the Order of Hermes might change. What if the length of apprenticeships is more variable? What if longevity rituals are less effective? Or many of the breaktroughs that are integrated into Magic Theory in 5th edition, like binding of familiars?
This would nicely establish a seperate niche for 6th edition so it doesn't compete as much with 5th. Even better, a lot of the material from 5th would be usable as inspiration for events happening in a 6th edition saga.
Anyway, just my random musings. 
Sub-Rosa #16 was dedicated to exploring the possibilities of earlier starting dates including 1 pre-Hermetic saga idea,
I think that would be a great game to create. I would not call it 6th edition, but I would very much like to play it.
It would probably not have what made me love Ars Magica at first sight 25 years ago. One of the strengths of Bonisagus' theory was the arts which can be combined so flexibly, and that was what first appealed to me. I also imagine that the scholarly bent and long-term development was much less typical in the old hedge traditions, at least the studies would be rather different.
I reckon the AD800 Ars Magica game would be much closer to rank-and-file fantasy. Europe was less civilised, and the magi would more likely have to settle in the wilderness, with no established covenants and tribunals to lean on. It would be fun.
... but again ... if you want to do that, it would probably be better to develop Mythic Europe supplements to a simpler and more main stream system (or if necessary a new but simple system). The complexity of mechanics is Ars Magica's downfall, and Mythic Europe (be it the 9th or the 13th century) is a great world which can be made much more accessible.
the complexity if ars magica is also one of its primary features. it's like the saying in neurology- if the human brain were simple we would be unable to understand it. simplicity for the sake of simplicity can ruin a gaming system- I have certainly seen it happen before.
In itâs 1st and 2nd editions, Ars Magica was not a relatively complicated game. It became more complicated, relative to other games, as the game developed more detail and options, while conversely game design in other games became more simple in the mechanical detail.
As such, I do not regard the complexity of the game as itâs primary feature - I regard it as something that needs to be looked at to get to the primary feature of the game. For me, this lies in having a magic system that allows you to develop your own style, and building story driven games centered around highly variable wizards.
I don't personally have an issue with complexity as such, but a game which none of my mates will play because it is too complex for them, has little value to me.
The value is hardly complexity in itself, either, but the stories it enables. It is worth a try to see what can be recreated with lower complexity.
To be honest, I don't think Ars Magica 5th edition is a terribly complicated game.
It uses a single type of roll (the d10) and you always want to roll high. This already makes it simpler than That Other Game and it's various spawn.
Admittedly, you can read that d10 in two different ways (simple vs. stress dice), but it's still not a terribly complicated mechanic.
So, where's the complexity? The laundry list of modifiers available? Maybe.
The lab section? That has turned people off before. 5th edition tried to unify it - thus simplifying it.
The huge libraries we see now? They were made necessary by the (community) demand for Arts based on XPs.
Maybe it's just ArM's reputation for complexity?
I've seen a lot of suggestions for using the FATE system instead, but FATE is more complex than ArM5 or eg GURPS, because it has than awkward 'negotiation' step to essentially every roll.
Still, I sometimes miss the simpler times of the 2nd edition too.
FATE is a great idea and an horrible execution. Aspects are great, but then it has a lot of layers of "regular" mechanics that detract heavily from the potential of Aspects.
ArM5 has A LOT of subsystems. It has a rule for everything. It is not that each rule is complex individually, but the level of rules clutter is massive. This is what makes it complex.
The first thing for me would be to reduce the power of hermetic magi heavily and to make everything equal across a type of activity. So all magic is a Te+Fo combo for example, be it for demons, faeries, magi, hedgies or other magical practitioners.
I would also gloss about A LOT of stuff. too many rules for minutae. Do we really need a system to calculate Apothecary production points? Really? really really?
This is a clever idea.
The character generation process, and the general presentation of character abilities is exceptionally complicated compared to most prominent game systems currently around today, and conversely itâs also significantly more complicated than previous editions too.
The combat system is more driven by wargaming standards rather than narrative standards, and the core âstress diceâ mechanic is slightly eccentric - with limited guidance on how many botch dice to roll should a â0â be rolled.
The magic system is necessarily more involved because this is the focus of the game, but this doesnât mean the rest of the sub-systems need the detail they have.
And for sure, I donât think Fate is the answer although the talk of possibly having a version of the game based on the Gumshoe rules could work I think.