[WARNING: RANT; Houserule]
D&D has two things going for it, that greatly ease the load of the DM/SG when building adventures or campaigns/sagas.
First - D&D is bigger. Much, much bigger, now that it's gone all open-source. This means more ready-made adventures. On RPGNOW.COM, I count 1246 (!) D&D3.x adventures, and 19 for 4e; I'm there are more adventures out there. And I'm not even counting all the adventures from previous editions and highly similar games you can easily plunder. You similarly have a huge stock of monster books, and even individual encounters and set pieces. So if you're strapped for time, or inspiration, you will find many more resources to help you start the game. Even entire ready-made campaigns, if you want to.
Ars Magica will never have that. It's a niche market. Which means that each SG pretty much has to design his own adventures. We might share more in terms of monsters, though even there I think the official bad guys list is not broad enough to fill out a campaign, because Ars Magica is too broad. Which leads me to...
Second - D&D has an expected adventure-arc structure, that is strongly supported by its rules. Especially since 3rd Edition, the characters are built to withstand and be effective against monsters level-by-level, and the campaign is to progress from fighting low-level to fighting high-level monsters. There are even fairly clear guidelines about how many monsters you can send after your PCs to challenge but not kill them. Building an adventure becomes an exercise in looking through the monster manual, picking up level-appropriate monsters you want to use, and doing some math; by then the adventure pretty much writes itself. Building a campaign becomes a matter of leafing through the manual to look up higher-end monsters and think up the "ladder" of monsters your PCs will face. Easy as cake. [Exaggerating a little, but still.]
Ars Magica currently doesn't have that. But I'm wondering if it shouldn't.
D&D is capable of pulling it off because it's essentially a superheroes game that is based on one primary stat - the class level. As you increase your class level your attacks and damage per attack increases, but so do your defenses against attacks and your hit points so it balances out. The rest is details.
Ars Magica can fairly easily adopt a similar mechanic. I'd use Might. For magi, tie Might to their Art scores. Have Might serve as the basis for Magic Resistance and increase Soak and defenses.
High Arts already greatly increase your damage output, and do so pretty predictably - a +5 increase means +5 damage; roughly. If it would also mean +5 Soak or +5 MR, we'll be a long way towards achieving this goal. So, perhaps:
Might = highest (Te + Fo) combination.
Soak bonus = Might. If all damage is Might-soaked - no equipment damage (from fire etc.).
If no Soak applies to spell/attack, MR = Might instead.
There is fine balancing to do and specializations and Houses to consider. And you'll need to redesign monsters too, of course, with this baseline in mind. But when this work is done - you'll have your Ars Magica superhero game. You will be able to know what the Might of creatures to pit your PCs against, and work from there, just like in D&D.
Now a big problem with this idea, is that ArM is currently not a superhero game, and perhaps many would not want it to become one. Still, I think superheroes are better then paper tigers - magi that can topple cities with a flick of the finger, but fall to the slightest offense they didn't prepare a ward against. This idea can serve as a basic principle for 6e.
Yair (ranting instead of designing his next saga)