I know that this is unorthodox, but here goes:
I think that the core rulebook should be split into two: one focussing on rules (which would have a little bit more of the the different Realm books, Grogs and Covenant added) and one focussing on hermetic mages. While in principle one could just make a bigger book, I think two 144 page books are more practical at the gaming table (if in person gaming ever becomes a thing again) as it can be more easily shared between players/story teller.
The rulebook:
The core book should include more help for new storytellers*, Chapter 16 is hidden at the every end of the book and is comparatively short. I feel like more advice should be dispensed for fresh story tellers, as adventure design for ArM is very different from the standard dungeon crawler style RPGs (which boil down to series of fights and convenient calibration levels so that the fight can be "fair"). Maybe having a preset adventure to give people an idea of what the game might be about and what to plan for.
In general I find that there is too little information on any of the realms in the core book, too little animal stats, too little on faeries... In general chapters 12 onwards are too short to be of much use.
This would represent chapters 1,3,4,5,6 and 10-16. The Hermetic virtues/flaws should be contained
The Mage book:
This would cover Chapters 2, some of 4 and 7-9. However going into a lot more details about what a mage is. It would contain the virtue/flaws which are mage specific.
The Core book is too light on information on the OoH, the code of Hermes, quaestior positions/duties, how a Tribunal might run. All this should be in the main book, rather than hidden away in the house chapters of various houses (or in the Tribunal's case in the Rhine book).The basics for the mystery houses should also be there (for example, there are next to no animal stats in the main book, which kind of sucks for a potential Bjornaer who might want stats for their animal form). I realise that much of this addressed in the further expansion books, but if people want to just try the game it's fine buying the core book, but asking them to buy 3 Mage books, 4 realm books, one tribunal book etc makes it a really large investment. One need only check the ars magica subreddit to find people asking which books to buy.
I think that overall the base books should be made more accessible to new players, which would help the community grow. I am quite aware that more information on the infernal realm in the core book is of little use to a grognard who owns every infernal book since 2nd ed, but more players would be nice.
*I am working on tidying up the notes from my own Rhine Gorge Saga, and to make them accessible, to help new storytellers to get started, as well as capitalising on all the reading that I did as part of my prep.