For those unfamiliar with the Ars Magica PnP system, can you briefly present it and point out its main strengths? How faithfully do you intend to reproduce these strengths and the PnP system in general, including the fatigue-based magic system, in your game?
This is a tough question. How to distill the magic, history, role-playing, alchemy, mystery, faith and personality of the table-top game? It defies classification.
In short, Ars Magica is a role-playing game set in Medieval history, in which you chart the rise and fall (and perhaps rise again) of a Covenant of mages and their underlings. The forces of the supernatural surround you: faeries, demons, magical beasts, and the encroaching world of the mundanes and the Dominion. Every character in the Covenant has a life of their own, with likes and dislikes of their own, and you can play any of them when you need them. Their personalities grow as the years pass, and by the end of every Ars Magica Saga you usually have a story or two to tell about everyone, even the least peasant. The heart of the Covenant is the mages, and each of them are vying for greater knowledge, power and prestige. The magic system is pseudo-scientific, and applies an interesting rigor to spells which you see virtually nowhere else. Your mages can research, study, and distill the forces of magic, creating ever more powerful, useful, or fun spells. Furthermore, via Spontaneous Magic, you can create a Spell to do most anything within the limit of your ability. We could go on and on, but that hits all the key topics. : )
We strongly feel that magic is an omnipresent part of the Ars Magica system, so it's omnipresent in our game, too. Whether you are looking at the Covenant map, in an Adventure, or a Combat, the spells which are contextual to the situation are available for use. And if you don't know the Spell, you can always try it Spontaneously. The Fatigue mechanic (and Twilight, too) are present, per the 5E rules.