Magic is always a pain in the ass. How do you balance it, how do you differentiate it, how do you keep it under control: these all wind up either driving a boring system or a subsystem with all the complexity that implies. Games that are about magic fare better since you don’t need to differentiate it at all and you can go berserk with the system.
So Soft Horizon is obviously tending towards the simple. A subsystem is right out. So how can we manipulate the existing system, following the style rules of change being additive and contextual, to get a magic system?
First off, anything can be magical by describing it as magical. This is the undifferentiated method: magic is just magical narration. This is fine if you want to magical incidentally in the world. But what if the world demands magic? Then you want something meatier. So let’s balance effect and cost to get something we didn’t get before. And let’s try to make it get better in a useful fashion without adding too many new rules.
We’ll lean on risk. In Soft Horizon games every conflict has a risk. And we’ll make magic a specialization. So if you add a specialization with the prefix sorcerous (say you specialize violence with sorcerous six-shooter) then that specialization starts at d8 instead of the usual d6. That’s your magical benefit: bigger die. And the narrative oomph of having a sorcerous revolver of course.
Now the cost. We’ll define sorcerous as running the additional risk of self-harm. So whenever you bring that die into play, you add harm to the existing risk for the conflict. If it was originally waste, now it’s waste and harm. If you screw up, you get injured in addition to all the other crap that goes down. Just to cover special cases, if the risk was already harm, now its scope expands: everyone in your group gets wounded.
Of course as soon as you build a special case you want to generalize. So what do the other risks look like as schools of magic?
Spillover. Demonic magic. Every time you use magic innocent people get hurt. If that’s already a risk then you cause a plague.
Waste. Well you gotta draw your power from somewhere. Call down a pillar of fire on that swarm of scarabs and if the risk gets realized, it uses all the gas in your car and the air in its tires.
Revelation. A seer or oracle perhaps. When you weave your magic you have visions and they are always bad and true.
…and so on. They all work. Of course if you roll well the risk is not realized. You get your effect without paying for it. And as your die gets bigger, the chance of this goes down. So advancement rules as they already exist take care of reducing negative effects of your magic.