If you have not checked out the thread on Discussing the Gen Con 2015 panel yet, please be advised this topic builds on that one so it would be good to read Timothy's summary at least.
Let me paint you a hypothetical scenario. Let's say Atlas decides to develop a new edition of Ars Magica. Let's say this new edition will start with a single core rule book with the same page count as ArM5. This will have to be a complete, user-friendly orientation to Ars Magica, Mythic Europe, troupe style play, covenant building, storytelling and roleplaying, and all the mechanics you need to run the best and longest saga you've ever had.
Let us also say that you have a friend who is a great GM and is all excited to launch a new saga using this new core book and ONLY this new core book. She really wants you to join that saga, so what is in this book is going to be your entire Ars Magica experience if you accept the invitation.
Apart from the obvious -- an explanation of the Techniques and Forms, how to make a character, a list of spells -- what do you think MUST be in that book? And if you are adding something that is not in the ArM5 core book, what would you cut to make room for it?
It's all well and good to say "we need gruagachan in the core book" (for example) but the challenge from an author's and publisher's point of view is that there is not time and budget and page count to fill everyone's wish list. So what would you say is essential to making Ars Magica great, and what is perhaps important to you but able to be sacrificed to make room for what's really essential?