One magic system I've always found would be interesting in an rpg would be Michael Moorcocks elric of melnibone. Specificaly having to depend on pacts with demons, gods and other supernatural creatures in order to obtain magic items or spells.
In Darker than Black there are Contractors. They have a "contract" which means they can cast one particular spell, after which they have to complete their "contract payment" before they can cast again. That's often a simple but annoying task like smoking a cigarette or writing a poem. The spell and payment are different for each one. Nobody got to choose.
That is such an interesting idea, especially because so many of those "obeisances" mimic OCD symptoms.
You might really like Ron Edward's RPG Sorcerer, and in particular, a later supplement book he wrote focusing (in the sense that a love letter focuses) on the sword-and-sorcery genre, Sorcerer and Sword. The game focuses laser-sharp on getting power from dangerous, mysterious entities.