So I ran a Magic roleplaying game by straight-up tacking Magic: the Gathering onto FATE. We had a good time.
I should've stopped at bridging the gap between FATE and Magic: the Gathering, but I didn't. This thing is a sprawling, 32-page document that creates a new set of Magic concepts and systems which absolutely do work as roleplaying background, but are fucking impenetrable. But hey, it's 17k pages so I'm gonna share it with folks.
The real conflict in creating a Magic RPG is making the cards work in a roleplaying setting. I did that by instituting a set of power tiers--called moxes--which allow spells to scale in power with the planeswalker (or creature) casting them. In the lowest mox--Mox Alpha--most spells take on a benign form. Tapping down a creature can make them drowsy, a burn spell can provide light, and a creature might become confused or addled if you make it discard a card. At Mox Unlimited, a planeswalker can cast spells which annihilate whole armies.
The FATE integration isn't that strong, but one of the options is non-combat casting. Five skills (Will, Investigation, Deceive, Rapport, and Athletics) are aligned with each of the five colors. Players may discard a card to invoke an aspect for a skill check instead of spending a Fate point. One of the colors in the card's color identity has to correspond to that skill.
The Tempest is basically a way to rationalize the randomness of a player's deck.
Planeswalkers channel their Tempest which provides them with a channel of aether which they can shape into spells. When a planeswalker runs out of ways to turn their Tempest into spells, they must revert to Mox Alpha or die by decking out. The Primer focuses a lot on reconciling these things.
Planeswalkers have fetters, which exist to explain some planeswalkers having higher "starting life" and to give players a potential ways to connect to planes. As is, fetters are just a way for powerful NPCs to be even more powerful. It's a bit much, but it's not like anyone has to use it.
I'm especially proud of the three-page "Life, the Multiverse, and Everything" appendix at the end which conjectures on how planes are formed.