Mechanics and flavor design principles regarding magic never made it to either of my initial lists, in large part because I was drawing from the broader body of cyberpunk literature and not just Shadowrun. Obviously, there weren't mages and spirits running around any of the other stories I considered. Still, it's a fundamental part of the game, and my party has a mage in it.
Shadowrun's spellcasting is of a particular flavor, and in order for it to feel like Shadowrun, these things need to be represented. In my mind, the two biggies are drain and overcasting, in that order.
The big problem with drain isn't the concept, but how to avoid extra die rolls. I've got offense and defense in combat down to a single roll of the dice; I don't want every spell to require two die rolls just because it's magic.
The easiest thing to do, both for me in design work and players learning the game, would be to take what I've got and apply it here. That is, your spellcasting successes get divided into yang dice, which power the spell, and yin dice, which resist drain. In fact, this gets easier yet, because with this setup, I can likely use Shadowrun's grimior, and the drain formula for each spell. So mages can select the force of their spell, with the force determining the base power of the spell, and the same roll resolves both casting and drain. However, because a player can split the dice as he likes, there's a small tactical element to casting. It's an additional step, but I like giving the player that control.
This leads to the issue of overcasting and pretty much solves it at the same time: use existing SR rules. That is, if you cast at a force equal to or less than your magic, drain is stun. If you go over, it's physical. Simple enough, and very easy for existing players to make the transition.
The one wrinkle is if a mage wants to dodge incoming attacks and cast a spell. I'm tempted to say yin dice can do double duty on this. So any yin dice spent to resist drain aren't used up; they can also be used to defend against an attack. Though in this case, the mage would have to combine dodge into his pool to use them that way. I don't want to make a player split his dice three ways (spell power, drain resistance, and physical defense) because with the goal of keeping die pools low, a three way split is brutal.