No, don´t mesh Ars Magica with Fate!
I love both systems, but for completely different reasons.
If you´re adopting the Fate Rules upon Mythic Europe you´ll lose many of the qualities Ars Magica has. In addition, Ars Magica would be "one of those Fate Settings" beside many more (they have ca. 30 official settings or offshoots already). And to say the least: The most Fate settings are more surprising, more dernier cri and have more pop appeal than Mythic Europe (I have to say it, even though I love Mythic Europe).
If I´d want to lead Ars Magica out of the present downside trend, I´d do two things:
First I´d examine as accurately as possible what new systems have evolved and what it is that makes them successful.
In my eyes with the 5th edition Ars Magica removed itself from the storytelling systems. I don´t lament about this, but if you´re looking at the new successful systems the most of them use some roleplaying devices, that strengthen the storytelling share of roleplaying.
Look at the aspects and the player empowerment of Fate, the Bennies of Savage Worlds or the Freeform Style of Fiasco! Ars Magica has some of this elements (confidence, spontaneous magic), but either they are not very important for the overall impression of the game or they are perfected in a manner that surmounts the complexity of the most new systems by far. In my regional roleplaying forum Ars Magica counts as a system that becomes old. The reason is the absence of most of this new hip mechanics.
Second, I´d create a think tank with the best authors I can get (and this will be no problem: Ars Magica already has the best authors). The task: Don´t try to imitate the new ruleslight systems. Try to lead Ars Magica to the future in a surprising way. You have to develop some new mechanisms, that the people will talk about. This mechanisms should not be developed to climb on the bandwagon, they should be developed with an eye for Mythic Europe and the assets of Ars Magica. I think, it is possible to retrieve the storytelling aura of the game. But you can´t do it like 2nd or 3rd edition. Keep in mind, that today there are systems like Fate! You have to find a new way! Once upon a time Ars Magica was the vanguard of the roleplaying systems. Try to reconquer this place.
The roleplaying society in my country (germany) is divided: Some of them love the new "systems to go". This is not your first client base. But the others still cling to D&D4, Gurps, whatever. Slowly they feel old, because the new revolutionaries poke fun at their slow systems, complex mathematics and heavy rulebooks. I think, they´d be eager to get a game, that is modern and fits their taste of roleplaying.
Don´t set the approach of precision that is typical for Ars Magica at nought. If you lose this, you´ve lost the game completely. Apart from that, be bold, permissive and fearless! It isn´t very important if there is a 6th edition in 2016 or 2018. What is more important: This edition has to be a bomb!
I believe in you.
Chiarina.