Books I would like to see.
(This is blue-sky thinking. Nothing in this topic should be considered to oblige Atlas Games, of course.)
A Rules Cyclopedia - not all rules, not anything from highly optional material, but the main stuff in one book.
Tribunal books for Iberia, Rome, and Loch Leglean, to update the previous books. Those books are old enough to be in college, and out of timeline too.
A master spell book with guidelines, shape and material bonuses, device effects, etc.- effectively a Wizard's Grimoire. I suspect there would be enough material to be separate from the Cyclopedia, above.
A Companions book (although I will admit Grogs stands in for this fairly well).
Alternate Ars books-
After the Founding
Late Middle Ages
Rebirth (say, 1410)
An alternate histories book - there could be several of these.
There could be locations books on India, the Far East, based on travelogues of the period, or on Mythic History of the location, and both (travelogue-based books could be set in Regios.)
I suspect that Saga books could work - long term outlines for Sagas, starting with freshly Gauntleted magi, with guidelines for what resources to make available and what to rule out. This could be more effort than it is worth, though. I am always skeptical of detailed adventures for Ars Magica.