Yeah that is difficult and I can see the whole intense thought discussion.
I will say that for me the most interesting things are magic (expansions of current options in both a 'canon' and an 'optional' way), spells and rituals, enchanted items, familiar options, characters, locations, and adventure seeds. On the last I totally think some short, say one session, some medium (say a few sessions), and occasionally an epic campaign wide saga seed could be interesting.
To me the dual fact that its an official work and that it is a consistent source of new material could make the possibility of signing up a possibility to consider.
Its a very interesting thing. I will say that I am super glad that Ars Magica is not 'dead' as the whole 'closing of the edition' made it seem. That was a sad day indeed and so hearing that there is potential for more, even if there is not a decision on what that 'more' is, is brilliant.
I own all the books and I will say that the location (both the Tribunal and the Realm ones) books always get pulled out the moment I am reading about an area. I also pull out the various adventure books when I need random ideas. Then comes the magic books (the ones that have new traditions and alternate ideas) which I often read for pure fun.
If you were wondering about what to do as an immediate jumpstart then I think updating Tribunal books to current edition (as this thread represents) would be a great start.
Anyway, this is awesome and I am soooooooooooooo glad that you guys are considering doing new stuff for the line.
PS. I totally think there is fun potential in material that could potentially change the 'world' in ways that are one off or unique. Specialized magic whose discovery changes things. Alternate realms that make things go "woah". Timeline changes like setting things a bit before or after or whatnot.