Hi,
Well, for that game, I might recommend using a completely different system, a lightweight system. Say, Over the Edge.
Three positive traits; for magi, one of them is your House.
AM is a rather heavyweight system, so a lot of preliminary paperwork has been put into those 2 rounds!
Anyway,
Ken
I would agree on an alternate combat system for fights with no magi. Might make it more fun than the one mistake and death that is the current system.
For JL...Is the problem in combat or out of combat with the casting of mastered spells multiple times. It seems like out of combat( or off camera) but perhaps I am reading it wrong.
nu-fatigue for spellcasting feels like it would drive us to the 'Scribe scroll + Wand of Cure Light Wounds' D&D model. You put your Pilum into an enchanted item (wand), then the wand casts the spell the same way round after round with no worry. On the other hand, if items also incur the "fatigue", then what happens to current in game multi-spell items, magically trapped sanctums, etc etc?
If everything magical can cause nu-F, I think it would be a substantial change but could work. You would sorta be adding Shadowrun's "background count".
Idea along those lines: Every time any magical effect above, say, second magnitude happens, increase the botch dice. So dragon breathing fire, you casting a pilum, or your enemy casting a pilum would all raise the botch for everyone. Goes down one die for every blah (diameter?) without any magic happening. What defines a Hoplite now isn't necessarily knowing Pilum or having X penetration, it is being able to last at least X rounds in a fight where magic is going every which way before your spells start exploding in your face. Vim spells gain a new use of manipulating the background count.
A possible benefit of Verditius would be that their casting tools partially shield them from this effect. So you might see more Verdi non-lab-rats. Talismans and familliars would probably have a similar benefit.
But then I would start wondering about Tremere, with their 'wizard soldier' mindset and their liking of spells multi-cast to buff your grogs or to wipe out masses of enemy grogs. What tricks do they have, and does Certamen figure into it at all?
Jebrick, both are concerns.
Im less than satisfied with the often binary result that happens with magic in practice. I've already said I don't care for the Monte Carlo approach to relaxed casting of mastered spells.
Items aren't something I'm generally concerned about. Items represent a huge investment in vis and time. The wand of fireballs has to split a lab total into uses per day and penetration. They can be countered.
The real risk of this model is that the best Hoplites will be mundanes with True Faith and actual spear and shield. I wouldn't so much mind, but it may not be what people are looking for!
I do not see how you solve the on camera problem without removing multicast. Unless you change multicast to either add fatigue or add botch dice that ignores the mastered spell.
Off camera is as simple as banning it in game. Or allow it if it takes a season of time.
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Jonathan, I do agree with some parts of what you say, but not the premises.
You say "fatigue is bad in ars, let's find a solution" (or at least that's what I understood). I merely say: "all which is used in combat is not really good in Ars magica, starting with the first to shoot win" (and fatigue is finally just one part of that problem : I have seen long battle where fighters never tire because fatigue rolls are not well defined. I have seen fighters with 36 weapons but no damn problem due to load. I have seen 1 fighter so good that even 5 enemies couldn't break his armor or even hit him due to the in existence of a "surrounded" mechanism. I have seen NPC magi be owned by one good roll on a sleep spell. I have seen PC magi be normally dead because they can't react to anything they don't perceive and had to provide a chance for them to dodge/try to fast cast...).
I'm not really fond of the "first shooter wins" type of magic in Ars and I acknowledge fully that if a NPC comes to combat, then he was stupid, because magic in Ars is more about "finding the right arcane connection, then fix it in your lab, then invent the charged item with +100 penetration at arcane range, then fire." which is lame for most players who will think "wizards => magic => battle spells => wonderful magical battles in sky".
I try a lot to have NPC-treats be powerful, but when they are, this is not fun for PCs because they cannot win. If I do "less powerful" NPCs, then those NPC are, almost always, by design, flawed to win. I have a really hard time to invent an antagonist which will not give either a full victory on the PC side or a full victory on the NPC side. So I'm using the environment as a mitigating factor, which fatigue is... but I'm confronted with the fact that fatigue is, in itself, badly designed in Ars magica.
Which is why, I agree with you that it needs rewrite as part of the combat process... but not alone. Alone, it will not fix all the issue I have.
in Ars combat situation, I have never seen any smart player - or if you prefer "not beginner and used to the game" - use any spell which would end in a gain of fatigue. Even when they roll 0 and do not botch, they always think ahead to avoid this and so only use formulaic spell with a spare casting total for auras and other bad things. When fatigue happens it is mostly because the situation is lost, and magi are trying to save what they can from a disastrous situation. And mostly, not themselves - they all can teleport away or fly or other means to flee - but the companions, who would be doomed otherwise. That's the only rare occasion when fatigue happens in combat, and at that point, it is irrelevant because they are already wounded or disappointed.
I have no problem with fatigue outside combat, since it all comes back to the main question: is this stressful or not, which is something the SG fully controls by the rule. There is no house rule in there. Obviously in game, I cannot always answer "it is stressful" because players are cunning enough to give other players time to act, then come back to me saying "after those times magus XX did what he did, I spent this time resting". And since I do not note what every magus do at every hour of the in game time.. I let them do. Only just before a fight will I be attentive because preparation should be something important. But again, most fight-spells are formulaic because penetration needs or... because you don't want to be fatigued.
But again, if your new idea encompass all the fatigue mechanic and is better in and out of combat, I'm interested.
So yes please, for the mind exercice, find a new fatigue mechanic and why not find a new combat design - and i'll read it with interest -, but anyway it will not matter since this edition is done, and I really don't plan on buying a next one because our players are now accustomed to this one (and I spent a lot of money completing this edition). So unless it is an official erratum [size=85][and even with that... errata which are only on the websites are lost to all the players in the heat of the action because they didn't write those in their paper books (and I'm not even mentionning the fact that in our IRL troupe, I'm the only one who really reads the errata and almost the only one which gives credit to the forum contributors here[/size] [size=50]which is their loss but I cannot improve their way of thinking[/size][size=85])... Sometimes I do remember an erratum and refer to it during a game session, but since I do not use tablets or such thing, if my memory fail, then no-one will know something is errated][/size], it has few chance to be of use for me in my RL saga where the alpha storyguide has his own idea and is quick to decide what he likes or not, period and is not the type to be discussed with.
In my previous post I merely explained how I acted and tried - but apparently poorly ... sorry but it is not easy to think in English 
- to explain why "fatigue in itself" is not a problem for me because my problem lies a level above so fixing a part of that is not going to help all.
I'm lucky in that my group didn't decide to build a big combat system - we have a Verditius with noncombatant, a Jerbiton, a Tremere socialite, and an Ex Misc investigator; the last being the most combat capable.
I certainly agree with most of your points here: The way to deal with mages being overpowered in combat (caveat: if the mage is built for combat) is to make it so they have to deal with other things before combat. Or to change the combat design formulas around so that it becomes less of the same: Four seperate smaller faerie murderers instead of one giant; Make the bad guys more set up; Don't let the players know exactly what they're getting into, due to demonic influence or other wizards' manipulations. Perhaps the way your mages deal with a problem is to get an Arcane Connection and go home, fix the connection and build a death-laser with massive penetration. I kind of wonder how many farms will be burned down in the meantime, but that's probably because I motivate my players with collateral damage.
Though I certainly understand the idea behind this, it does take away one of the primary benefits of mastering the spells.
Certainly.
But in our saga, one of the house rule of our alphasg is : "magical auras add to botch dice" and another is "starting number of botch dice is 3 for everything". So mastery is very important anyway.
Lucky me. The general rule my players is 'convince the other players to overextend and botch'
In combat, I'm not a fan of the often binary result of success or failure. Do you penetrate? Yes, you win. No, you lose. Granted, it's not always this simple, and while I do find stories about the consequences of combat interesting, I still find the combat to anti-climactic. If you can't penetrate with your best spells, there are almost no options, unless you've taken Life Boost.
Outside of combat, because Hermetic Magi can unleash massive power, especially power to build, they can do a lot during the season. The Monte Carlo simulation isn't really a practical issue, but is something that is possible within the theorycraft of Ars Magica. I don't like the possibility that someone could just do something until they get it right, when they have no chance without actually exploding (on the die roll)...
Edited because of sarcasm.
Yeah,
I don't think people would recommend practicing piano if there was a good chance you'd explode before getting it right.... 
/2
Anyway,
Ken
Just make fatigueloss by spellcasting long term fatigue..
That isn't a solution. How many magi know spells that they can't cost with fatigue loss? My experience suggests that's rare. It might happen with a lab rat who has assistants and a high magic theory score. So it only impacts repeated use of spontaneous spells. Spontaneous spells already have something of an effective limit.
You would make something like Endurance in the Hero System. Each Magus has X amount of Endurance based on some base * stamina score ( plus or minus any virtues or flaws). Each spell costs y ( perhaps modified by it's magnitude ). Keep doing X-y until they cannot cast spell. Now the Hero system has a way to gain some Endurance back each round. For Ars you might say that you get recovery after 4 combat rounds.
To expand on jebrick, and take a cue from some books/comics
Magi have 2 sources of power: their own internal energy, and the surrounding one.
You can have a mage cast freely any spell whose magnitude is at most equal to 1 or 2 times the aura bonus s/he gets. In short, the power comes from the aura.
If the aura ain't enough, one can either use vis or one's own energies (fatigue), at the cost of 1 pawn/fatigue per (2 to 5) magnitude.
This doesn't eliminate the problem, but it alleviates it somewhat.
If you want to cripple magi, you can require them to "pay for" negative magnitudes in hostiles aura (1 pawn/fatigue per -1). This makes infernal, and especially divine auras, very debilitating to magi, which may suit some sagas (encroaching dominion, dwindling magic, life-draining infernal power...)
I've been working on a bunch of House Rules and among them is a rule like this, providing the caster with a +5 bonus to their casting total at the cost of fatigue. Life Boost allows the caster to spend any number of points of fatigue for this process and Mythic Blood provides a +5 bonus to casting totals whenever the caster spends fatigue to boost their casting total (with only a single +5 being given no matter how much fatigue is spent on a single roll).
It helps alleviate the binary nature of the spellcasting, gives the magus something meaningful to spend their fatigue on and helps eliminate the undercasting rules for formulaic spells, which is all upsides in my eyes!
If you're doing a GURPS Ars Magica game and not porting over the magic rules, you're missing the primary draw of Ars Magica. I mean, the setting is okay, but the GURPS magic system is kind of boring and not nearly as fun to work with as the Ars one.
I've contemplated this myself, because I think the GURPS base rules are superior in most respects, but I've struggled to think of a "clean" way to convert the magic system.
The biggest challenge, I think, is scaling Experience appropriately. Maybe you could set things so that you only gain magic experience through seasonal activities as a sort of cheap dodge. Maybe you could estimate a Quality conversion to CP - unfortunately, that loses one of the nice things about GURPS, which is that you can do a more flexible time-based training schema since each CP = 200 hours of normal instruction.