How I rule soulfire i think is kinda cool(now watch me post it and you all take it to shreds lol) Okay so you all know how you can use sponsored magic to power up your magic by incuring debts? Well i play that when you use soulfire to make those you are actually causing consequences to your aspects(since for this game the best way to define soul is through them) At a rate of one mild consequence for every three shifts(i do three because soulfire is a little more limiting than regular consequences) of power gained. How i Rule it is it works like this...
Example one.
Soulfire User
Aspects: Chosen of God, Power of Soul, Soulfire is My Weapon.
Okay now this soulfire user hurls an attack and tosses in an extra 3 shifts of power from soulfire, this would be a mild consequence. So he looks at his aspects and says Hmm Well i dont think i will need my "Chosen of God" aspect. So he makes a mark on it. That aspect is no longer usuable for the duration of a mild consequence(one scene after its gained) that means no invokes or compels from it.
Example 2
Soulfire User
Aspects: Chosen of God, Power of Soul, Soulfire is My Weapon.
Okay so say he tosses in a huge amount of soul fire into a single attack which would be 9 shifts(a severe consequence's worth plus the 1.5x bonus for it being soulfire) Okay so he looks at his aspects and says well i almost never use my "Power of Soul" So he picks that and puts a mark on it. Now that aspect is gone like a severe consequence(so no invoke compels or anything like that) which means it could be gone for a very, very long time.
Example 3
Soulfire User
Aspects: Chosen of God, Power of Soul, Soulfire is My Weapon.
So he uses the same number of shifts as last example(9) which could be a severe consequence on one aspect. but that aspect will be gone for a quite a while(and thus he will lose lots of power) so maybe he decides instead of making it a severe on one of his aspects he divides it up into 3 different mild consequences and places them on his 3 aspects and they will only be gone for a Mild's duration.
This is just a house rule of mine and probably stupid but hey i like it. And after having played several sessions with a soulfire user i have yet to find any brokenness in it or Over Poweredness. Plus this represents how overuse can be fatal(if you sacrafice your high concept you die) Still tweaking it though. Right now one of the things im trying to decide is if your trouble counts as a sacrificable one(right now i'm ruling you cant.) Also i'm ruling that you can use whatever form of thaumaturgy at the speed of evocation you want with soulfire as long as you can justify it with your aspects.
So what you guys think? And please dont be too harsh