The stories in my saga tend to emerge organically out of the activities and prior relationships of the PCs or are related to the very particular circumstances of the covenant - so I have not found bolting in published scenarios to be feasible. I think this situation is pretty common.
What I can use is developed NPCs (such as are found in Antagonists and Magi of Hermes) and interesting locations (as found in Through the Aegis and Mythic Locations) - these are easier to drop into an ongoing saga.
But you are right that the range of possible settings for individual sagas makes it much harder to publish material that is relevant for a large proportion of the community - and this doesn't just affect adventures but all kinds of source material. With fifth edition there was a tendency to publish books at a high level of generality to try to get around this problem. This largely worked but could make the books a bit vague and abstract. The treatment of Faerie is a good case in point - while the old Faeries book was clearly rooted in the folk traditions and beliefs of north-western Europe and all the more compelling for it (although it would have been an odd fit for a saga set in the Theban or Levant Tribunals), with RoP: Faerie there is the sense that the legends of Greece and Spain should fit in with a conceptualisation of Faerie just as well as the tales of Brittany and Scotland. The book does a pretty good job of integrating a broad range of cultural material but can only do so at a high level of abstraction, which doesn't provide much direct inspiration for someone wanting to develop faerie stories in a particular cultural setting. Therefore ultimately it is a bit frustrating.
I think with sixth edition a different approach could be taken. Rather than trying to cover all of Mythic Europe equally, I would suggest that the line focuses in on a particular tribunal, or a pair of neighbouring tribunals. This would make it easier to publish a sequence of books which would all be relevant for anyone playing sixth. And accepting that a sixth edition line will never be as extensive as the fifth, it would enable the line to go into the setting in more depth than fifth simply because the books will be more geographically focused. This would actually make the line more attractive for people who already own all of fifth - they wold know they would be getting something knew, not just a rehashed version of material from fifth.
Sure we have tribunal books for fifth, but for me they don't give nearly enough information to run a decent saga. There is a reason Glorantha players keep going back to Sartar and Prax, and WFRP players keep going back to the Empire - these are highly developed settings in an otherwise much more thinly realised world, with a lot of established lore which experienced players have become fluent in. I think it is time Ars Magica had its Dragon Pass.
My preference would be for Rome (or Rome + Greater Alps) as Rome is at the heart of the Order and did not get a fifth edition tribunal book (there is a third edition book but it is pretty ropey). Others might prefer Stonehenge + Loch Leglean but there are already many ways of playing a historical fantasy RPG set in medieval Britain - it is pretty well trodden ground.