I want to draw parallels to witch and clerical powers in RPGs.
Clerics get their power from a deity, and witches get their power from the entity they make a pact with. The two are similar in this regard. Advanced Fighting Fantasy has an interesting approach to clerical magic.
In Advanced Fighting Fantasy, a priest gets a unique suite of powers based on the deity he is devoted to. Each god grants three powers from a list of generic priest spells. There are generic spells such as Heal, Smite (damage versus a particular type of creature), Banish (undead), Bless, Ward, Shape Change.
Each god also grants one power unique to that god. For example, Usrel, the Goddess of Peace grants a spell which creates a space wherein no one may take an aggressive action for a short period of time.
All the good gods grant the Heal spell, but only Usrel grants Banish. Another good god, Hamaskis (the god of learning) grants the priest a photographic memory, and a bonus to Lore special skill tests.
There are neutral gods that grant powers related to nature. This is good for running a Druid type character. Hydana grants the power to walk on water. Pangara lets the priest summon wind. There are Beast Lords which are the essence of a type of animal that grant the power to speak and be friendly with animals of that type eg. bears, deer, snakes, etc.
The evil gods predictably do not grant healing spells but some rather potent offensive spells. The god Decay grants the power to rot or rust one item per day, Disease grants the power to inflict illness, and Arhallogen the God of Spiders grants the power to be friendly with spiders and communicate with them. He also grants the generic power to shape change, so your priest can become a giant spider.
All the gods grant a one time salvation spell that saves the priest and possibly his companions from imminent danger.
All the gods require the priest to follow the path of that deity or lose his powers, just as in other RPGs.
The priests can invoke each spell power once per day, and if they pay a cost in Luck, can do so twice. The magnitude and duration of certain priest spell effects depends on their Priest Magic special skill, but otherwise thats it. It’s mechanically fairly simple but very flavorful. Even if you don’t want to play AFF, I suggest the game material as a source of inspiration in this regard.
Something like this might work well with Pits & Perils for the clerics devoted to the saints of various orders. Perhaps each Saint may grant a spell unique to their domain of authority under the Triune God.
I got to thinking that something like this might work well for Witches making pacts with various spirits. Those spirits might grant a small suite of spells and in return of course the Witch offers her blood to pay for them. And like a cleric, the witch would have to help fulfill the agenda of that entity to keep the pact magic. The entity may be satisfied with the witch spilling her own blood, but on the other hand, it may demand more.
I would need to playtest the witch to see how it works out. What I don’t want to do is step on the toes of either the magician or cleric class, but offer something in between.