I had a lot of good experience with Thrice-Told Tales and Hooks; mind you, I never got to the second stage of any of the Thrice-Told Tales, but I got a lot of mileage out of them regardless. (Because you go back to each adventure 3 times, TTT is the closest we have to an "Ars Magica Adventure Path.") Mythic Locations is also really great for this. Through the Aegis has built in adventure hooks which I stole. I love all these books. The longer adventures in Tales of Mythic Europe/Power were harder for me to use, but interesting reads and a great resource to be mined.
I would like to see adventures that illustrate some of the foundational adventure types in Ars Magica:
- Exploring an abandoned covenant. Calebais already does this, but that doesn't mean it's the only way to do this. In particular, I think exploring a Deidne covenant would be interesting and different. I realize ArM5 avoided the Deidne, but this is my wish list and I'm sticking to it.
- Vis hunt. Again, I realize this seems like a small story, but a one-session vis hunt may be the quintessential "what do you do in Ars Magica?" story, so let's show GMs how it's done when you do it well.
- The "Join or Die" dilemma. The PCs come face to face with coordinated hedge magicians, and must decide what to do about them.
On the topic of an "Ars Magica Adventure Path":
I've learned a lot about Adventure Paths in the last year and a half. I think something like this—probably not as a series of paperback adventures but as a single hardback "Saga in a book"—has tremendous potential. Not only do you show people what a whole Saga looks like, not only do you do most of the work for them in this work-intensive game, but you also create a shared "event" that Ars players can talk about. We see signs of this with major plot points that have already appeared in ArM, like the Lotharingian Tribunal, the Spider, the Duresca Scrolls, Mongol Invasion, or even the Deidne. Players love to share their version of what happened in their Saga regarding these plot points. But an AP would take that to 11, creating a whole community of people who could play the same campaign in real time, stream it, share their experiences on social media, create props and player aids for it, and do all the things that create and strengthen a community.
If it were me (and forgive me, I know it's not), I think it would be best to have a covenant location already baked in and start the PCs as they emerge from Gauntlet. There's two strategies here: Set it in Provence and let the PCs travel to all the various Tribunals we already have ArM5 source books for (the "This is Tuesday so it must be the Rhine" option), OR, and this is my preference, use this as an opportunity to explore one of the 3rd/4th edition Tribunal areas and update it as we go, giving new stat blocks for the magi we want to keep, going to locations we got wrong the first time (Sicily, for example) and revising the Hermetic aspects to fit ArM5 canon (Tremere). There are no shortages of good candidates. So, you could do the Mongol Invasion and update Novgorod in the process, or a Frederick II saga and update the Roman Tribunal, etc.
I honestly believe such a book has enormous potential for the game and the line.