Homebrew is one thing, modded is another. IMHO, I feel it is best to learn the actual rules before moving on to a modified system. Homebrew is created from scratch (in theory), so there is no confusion involved.
Simplified AM:
Problem 1: Magi character sheets are way too cluttered.
Solution: Magi can and must use the magnitude of Arts scores to replace skills. If a normal character needs a virtue to learn a skill, a magus must have that virtue to use Arts magnitude for that skill. A magus must have Gentle Gift to do this with social skills with anyone affected adversely by Gift. Well-travelled lets magi have a good feel for places. Etc.
Supernatural skills taken as usual.
Languages, based on Com.
I can expand this, if there is interest.
This is a stopgap measure for this edition; I'd want something like this better integrated should a new edition arise.
Problem 2: Cumbersome Library and Book Rules.
Magical libraries were simple back in AM3 and before. Now they are cumbersome and complex. Worse, they do not reflect library use or the relationship between scholars and important books. (RoP:D offers some particularly silly and offensive examples of this. Is the Koran really a higher quality book than the New Testament? (Not in my opinion, and I have read both.) Are rabbis who spend a lifetime studying the Talmud stupid, because it is only worth 6 tractatuseseses? (Um....)
Fortunately, AM5 book study can be modelled simply by saying how many xp per season it offers. 10xp/season is a good number. An SG should also decide what the state of the art is for each Art and Ability, leaving some effectively unlimited and others quite limited. Arts and Magic Theory should be unlimited; texts for most Mastered spells are limited and perhaps non-existent.
Rule--a character with a decent library can read for 10xp/season, if the library has texts pertaining to the subject.
Rules for library acquisition and maintenance can be developed as needed.
Problem 3: Too many die rolls.
Any defense against a rolled total is constant. This includes soak, dodging, parrying, magic resistance but explicitly not fast-cast 'defenses'. Whenever the rules call for a die roll, use 6 instead.
Finesse goes away. If a spell does something tricky, the spell must have a correspondingly high level. Spells can be directed but not Aimed.
This just about halves the dice rolling needed to play.
Anyway,
Ken
About too much randomness generated by dice: Use a d6 instead, with the 6 as a 0 for botches (if needed, I think they are) d8, d4, whatever strikes your fancy
The quality points are a tad oppressive, best to change them to/2 or something so people have a much greater incentive to change them (then a +1 on a stat will matter half the time)
Exposure 1xp is a good thing, but keep in mind that dividing everything by 5 will make magic a lot harder, though more flexible due to the flex formulaic. The way I see it people will be generalists more than specialists. No need to change the max arts, they will be a lot lower, not only due to the factor 5 xp division, but a higher factor due to new library rules. Since books are primarely about magic, maybe best to change the quality of books to 5
And as for vitues/flaws, they should be completely rewritten for this to work.
Still, I'd like to run this for a groupe, and see how it turns out
You realise the effect on commonality of botches i hope?
Which is one reason why its no longer "simple". 
Yes, I do. I like botches.
And no, it is not easy to set up, but for short games and light playing, arsmagica might appeal because of many of its aspects, but just be a tad clumsy to use.
If you manage well, you can start out with a few adventures with the simple magi and work them through, eventually using apprentices and the like to start a new game with similar feel in the RAW when much of the rules will be easier. Or just discard that option it entirely and have a lighter game.
Both are viable options, the system can become elegant, and it can become "upgradable" to standard ars magica, but I think the problem is how to do both.
By any chance, did you mange to playtest your system?
Being in need of a simplified rules set for Ars Magica, I would be quite interested in knowing how it worked over a saga.
No, as far as I know, the system has never been playtested... (I've been too busy with other stuff to do anything with it myself.) Feel free to try it out! The basics are there, but you'd need to develop some things further, in particular the spell guidelines.
FYI we have adopted a D6 system ourselves. 
We find that it makes your scores in abilities something relevant. We use additive sixes (6+6+6...) instead of explosive dice and you botch if you roll a 1 followed by another 1 (1/36 chance of botching). Works well enough for us.
Cheers,
Xavi
And by any chance do you know of other (simpler) systems used to play Ars Magica ?
For instance I have taken a look at the Chronica Feudalis system and while it looks quite interesting for the mundane skills (I especially like the conflict rules permitting to trade injuries for vigor and the character creation by mentors), I hardly see how it could be used to convey the same feeling as the Ars Magica system when dealing with magic.
We will convert ArM to Savage Worlds because some of our players have time to deal with the game only on the gaming sessions. Thus they didn't advance their characters since years.
The GM wants to convert the system of the Arts, too, but I think it's not necessary.
So you will be playing with a system for mundane interactions (Savage Worlds skills) and a system for magic (Ars Magica arts)? Don't you fear it will be a bit clumsy to use?
Maybe. The tests will show it.
I'll start off by confessing that I'm a complete noob to Ars Magica. My current, first, and only saga began this spring so while I may have nothing of value to contribute with respect to rules simplification, the experience of being thrown into ArM is still very fresh in my mind. Hardly a session goes by without us having to look up some obscure rule, like how does penetration work. 
Really. We look stuff like that up.
Three things have got us from utter confusion to the third year of our saga.
-
Awesome Story Guide. I don't mean that he knows the rules cold. He used to play a lot, but hasn't played much since ArM3. I mean that he has a great feel for the game and he house rules things in what seems like a fair way to keep the story moving. Between sessions, either he or I (noobie, but mildly obsessive
), will look up the relevant rule and bring it to the next session. He helped everyone get their characters started and has been guiding the game through progressively more complicated adventures. -
Start with role playing. For the first several weeks we hardly rolled a die. It was all talking to people and casting spells that we either knew already or that were well within our capabilities. Everyone played their magus and we all had a chance to try stuff out, knowing that our SG was not going to hold any mistakes against us.
-
The Undo Key. For the first two months the standing rule was that you could rejigger your character as much as you wanted between sessions. These were not the kind of changes that helped us deal with imminent threats or obstacles but the kind that let us build better, more interesting characters. One guy had no Magic Theory until he realized that it was part of the calculation for learning spells from lab texts. One guy (me) went from an Aq magus to an (Im) magus in one week then to a largely Vi magus the next week. This comes back to the SG. Ours just cares about having fun, so we were encouraged to take as long as needed to get the characters we wanted to play.
Confession:
We were not exactly thrown in to ArM with no preparation. We had just finished a two year Dungeons and Dragons adventure. DnD was good preparation for ArM, the way that high school chemistry is good preparation for university chemistry. It's not even remotely the same, but it gets you at least familiar with the right concepts. The one guy who hadn't played DnD with us is the one guy who dropped out of ArM.
We all got the gist of ArM right out of the gate. We knew roughly what we were supposed to do and we were comfortable with long RPG sessions (though cutting down to one die and then seldom rolling it took some getting used to). Even so, the rules of ArM seemed vast and dauntingly nebulous. "I can just cast any spell I want? What do you mean I don't have hit points? Where are the kobalds at?"
We figured out ArM and with the right SG so can every troupe. The rules don't seem so frightening now and I'm glad I'm not playing a simplified version of the game. We moved from DnD to get a more complex game. If ArM is too complicated for your troupe I'd switch to a simpler game before I monkeyed around with the rules for his one. Play for a while then have another crack at ArM.
That said, I'd like to second an earlier comment and say that your troupe is very lucky to have such a dedicated SG.
We have played Ars 5th edition for some years now and found the following flaws :
- the magi are quickly too powerful
- the system is to clumsy even though the tech + form idea and the lab activities are cool
We are thus thinking about implementing the following things in our next saga :
- arts as abilities (less powerful magi, only one numbering scale on character sheet, talisman and familiars much more useful, permit to more easily compute the basic score for formulaic spells)
- +1 point to effect gives only +1 penetration when creating artefacts (not to have them rendered too powerful by "arts as abilities")
- formulaic casting just requires spell level <= basic score (sta +form + tech + misc character bonuses) + aura modifier (without the die), never cost fatigue, should be much more simple to use, the die is only used for penetration computation (less than 10% of our spells usually) and botch risk
- spontaneous casting is spell level * 2, cost 1 fatigue
- only one vis type in lower quantity, in order to suffer much less from bookkeeping,
IIRC it was pointed out that the major problem with arts as abilities is the fact that the die roll becomes really important, and specialization does not pay off much, so generalists are much more powertful than specialists. A potential solution is to use D6 instead of D10, but the criticism still holds.
the problem with Techniques as difficult arts is that technique specialists become screwed. An intellego, or rego specialist is less playable than a herbam magus for example.
Someone proposed allowing an alternative to this consisting on fixed bonuses (or it might be my own evolution from someone's idea in this forums, cant remember). You started with all Arts at 0. They progressed as abuilities. However, you had to decided at character creation on how to get bonuses for a few abilities
You could get (IIRC):
+15 in a single art
+10 in 2 arts
+5 in 5 arts
+3 in all 15 arts
So the evolution of arts is quite fast at the start would but gets capped fairly fast by the simple fact that arts evolve as abilities. And you will always be stronger in a certain area. So you decide if you are going to be a generalist or a specialist and this is where your power lies. I really liked that idea.
Xavi
Now that one i can agree is simplifying. Although it also means loosing a good bit of flavour.
But especially for people starting out, it can be very nice.
A common HR that is pretty decent i think, even if i prefer to at least have the +2 left for Invested items.
YouĀ“re just changing one set of numbers for others, so no it doesnt?
Compared to how openended it is, its fairly simple IMO.
That, is a nice variant. Progressively larger total bonus as the individual bonus gets smaller, very neat.
Even for abilities, I dare say the die is already too important to my taste with the RAW.
I'm toying with the fact that formulaic spells could be launched without taking the die into account, just rolling it to check for a botch. The die would thus only be used for penetration only. As for spont, having a greater random factor is fine by me : I prefer my players to rely on formulaic spells and use spontaneous magic only with care.
Your idea of increasing specialisation (which you are right might be discouraged by arts growing like abilities) is really interesting. 
I might prefer implementing it through a virtue though, like "Puissant (Art)" made major. In conjunction with a magical focus it might be as powerful as flat bonus at creation.
In fact in our last campaign we spent a lot of time converting vis (using redcap rates) in the middle of adventures as we were annoyed converting it before. For multiple arts vis to be really interesting I think it must be managed precisely and micromanaged (in which case it can be a good start for stories), but we prefer to simply handwave this part as we do not make an interesting use of it.
Oh it does: computation is easier on lower numbers, and players can more easily keep the spells scores up-to-date if the forms and techs scores change less often (that's the main reason it is not done currently in our group).
No, it is complex, which is good as it allows for the integration of numerous factors (auras, familiars, character scores, etc.) that really add to the game flavour.
However it is also clumsy, with different scales, the magnitude/levels notions for spells, non-integrated guidelines across different forms (for instance there is no such simple thing as "level 10 PerdoForm to destroy the standard unit for the form") and multiple computations involved. There are also new systems all around the supplements (which happily tend more and more to rely on the core rules and not add new ones).
As an example, there are systems like Fudge where you just add a die for each help you get (could be D12 for your Creo, D10 for Ignem, D12 for your wand, D6 for the familiar, etc.) and check you have at least a die above the difficulty level instead of summing numbers.
No, compared to what you can do with it its quite simple.
I have "Puissant with" with Major version "Masterful with" where the major virtue gives twice the bonus.
I also changed those to always be added to the score, ie. they are included for teaching/writing etc as well.
So basically you pay triple the Virtue points to get double the bonus.
Added "Natural aptitude" and "Great aptitude" as the Virtues giving bonus "only when used"(as original Puissant) with the bonus being euqal to Puissant+1 and Masterful+1 respectively.
Almost never do that anyway. Usually easier to just count it whenever actually using a spell. Otherwise, once you have PC with several dozen spells, you will be spending more time changing the casting scores for them than playing.
If the difference had been between 2 and 3 or 4 digit numbers i might give you some credit for the claim, but you will be dealing with single and doubledigit numbers either way... How much harder is it to add together 20 and 15 than 10 and 7?
IĀ“m yet to encounter a person who is unable to add together two doubledigit numbers in their head, and if youĀ“re lazy or bad with numbers, you add a calculator as standard equipment when playing(actually a calculator should be standard euipment regardless, as thereĀ“s always lots of numbers moving around).
Still works far better than any attempt iĀ“ve seen to get rid of that difference.
Oh i WISH it was possible to come up with a few guidelines that could cover all forms. IĀ“m afraid it will skew the system very badly however. One way or another some forms will become too superior or inferior.
Yes, that's the main problem with that system. 
Really? I'm actually rather pleased with the complexity level of Ars Magica.
Enough fiddly bits to keep me interested after more than 15 years, but not differential equations like in GURPS.