I took the time to read this topic before offering an opinion. Many great things have been mentioned so far that I agree with:
- Streamline the rules in general to improve coherence. Simplify some, while preserving or even improving flexibility (combat).
- Focus on telling stories, keeping the book-keeping and calculations to a minimum during active play. I don't think people mind more complex calculations during off-camera time (like inventing spells), but during an action sequence that is intrusive.
- Provide a simpler way to create a character when starting play. The wall-of-complexity image is very telling here. Get people hooked up to the game before asking them to invest in hours of character creation.
- Keep the historical setting, that is part of what makes Ars different.
- Keep the Order of Hermes, it is also part of what makes Ars different. But get rid of some nailed down chamberpots, there are now too many of those.
About that last point, I personally found that about half of the material in the HoH books added constraints to how I saw different Houses, to the point that I no longer wanted to play them. Some because they were completely foreign to the setting (such as Criamon paths), others because they narrowed down the way magi of these Houses could be played (such as Verditius' Hubris).
I'm also dubious about the treatment the various Realms received in 5th edition. Though it had potential, the "Faeries being manifested stories" didn't echo at all with me. And the big "transcendent magical realm where you can transform" left me cold.
To me, Ars Magica should be about humanity, about how someone with greater power can use such power yet still remain fundamentally human. Whenever you start trying to exceed your fundamental humanity and become something else, you stop being the hero in the story.
Leaving philosophical musings aside, there is one element of the mechanics of the game, and that is how Fatigue is being used. I'd like to see Fatigue be used actively by the players during sequences, instead of the loss of Fatigue being a byproduct of actions. In combat, that means the player should be able to allocate an additional effort to attack in order to overwhelm an otherwise matched opponent, or to defense to fend off a stronger attacker and gain time for allies to come to his his help, or even to achieve a special result such as tripping the opponent. This could also apply with magic, to improve Penetration, to bolster his Parma Magica or to maintain concentration at a key moment. Make using fatigue mean something, giving the player a choice in when and how to use greater efforts to achieve a result.
Seasonal activities could also be reworked, depending on the level of commitment the player invest in them. Are you ready to commit greater efforts for a month, or for the whole season? You can achieve greater results. But if you need to interrupt your work something bad may happen (similar to experimentation results).
Get rid of simply scribed books on magical Arts. Books are fine for abilities, though the actual mechanics for them could change. But learning actual magic can only be done by being studying something supernatural. A magical creature, a faerie site, an enchanted object. All can provide you with new insights about magic. Magi can still create books that make studying of Arts easier, but these books are no longer a scribing projects, they are an enchantment project. You have to invest raw vis and magical resources to create them. Make each of them special.