I mentioned earlier that I had been hacking a number of systems together for my current campaign. For magic, I had borrowed wholesale from the original Ars Magica because it's still one of the most compelling magic systems out there and lends itself fairly easily to the cortex system and its basic methodology of multiple, often mis-matched dice.
The appeal of Ars Magica is mainly that it has an air of mystery and forbidden knowledge around the magical arts, creating a situation where the caster is truly a dispenser of esoteric and eldritch power rather than the D&D model, which tends to trend toward the banal in that casters become dispensers of certain pre-determined effects as the system's been played for so long by so many people that there's definitely an optimized "build" -- as an example, any first-level magic user who shows up without Sleep (or possibly Prismatic Spray in Type 2) is generally given the hairy eyeball.
So, here's how magic works in my hacky system:
There's a General Skill for Ars, with all the Ars Magica Techniques under it as specific skills and another for Morph, with all the Forms under it as specific skills. Each learned or granted spell (depending on whether you are a mage or a cleric) has one or more Techniques and Forms -- that I can just lift spells wholesale from Ars Magica is a big bonus to me. I do have to convert in terms of setting difficulty in relation to affect; however, this is a fun exercise as I can say that one spell can do anything from emitting a spark at your fingertips for low difficulty to setting multiple people on fire at a distance for a very high difficulty.
Each casting of a spell involves two rolls: first, the rolling of the lowest-die Ars and lowest-die Morph (ex: a Rego Auram Vim spell where Auram is d8 and Vim is d6 would use the latter) together and compared to the difficulty of the spell and then the lowest die involved in the first roll + Willpower (if a cleric is casting) or Intelligence (if it's a mage casting) versus the same difficulty. The first roll determines whether the spell succeeds. The second roll determines whether the caster takes damage from casting the spell -- the "fluff" is that you are forced to use your own life-essence to cast the spell as you could not draw enough magical energy from the surroundings. This damage is "aggravated" and thus can only be cured with rest or with other yet-undiscovered means. This damage goes straight to Wounds, so it serves as a decent limiter in terms of how often a mage might cast spells -- it's pretty easy to cast various low-difficulty utility spells over and over again, yet doing something powerful carries with it a considerable risk -- not only will you possibly fail at casting it, you might get seriously hurt or even die in the process.
As mentioned before, mages can attempt to "wild cast" by combining Forms and Techniques without the benefit of a learned spell, with the major caveat that it will increase the difficulty of the spell. Such attempts are uncommon at best unless the caster is in an especially magical area, which for the caster is determined by the nearness of a ley line, which add an additional dice to all spell casting rolls. Such areas are of course dangerous because of the people (or other beings) that are attracted to them.
Thus far, the system has worked decently so far -- the cleric has mainly failed with all of his spell rolls except for one and has only taken damage once (coincidentally, it was the same spell, which worked out fairly nicely thematically) and the mage has made almost all of his spell rolls while also having taken enough damage that he's become fairly leery of casting in the future, to the point where he has claimed he's done with playing spell casters after this character -- something that I expect to be revisited should he ever get access to an area with a ley line or heaven forfend, a nexus of them, which they almost did last session; however, they decided against a boat voyage into the southern swamp in favor of revisiting the secret entrance to the megadungeon, which ended with the dwarf on the wrong side of a closing door in an ancient drow crypt...