I like magic that's weird and complex and entwined with the setting in which it exists. Usually, I make something bespoke for every game I run - you can see an example here.
This is at odds with the standard D&D approach, where there's a list of somewhat arbitrary spells that get taken out of the box and used as-is, without much room for additional depth.
The local blogosphere is a bit better. There are lots of posts out there that think about what magic actually entails, or which go for an interesting aesthetic, or which have a solid thematic throughline.
There are two models I see a lot. The first conceives of spells as invisible critters that a wizard keeps locked up in their brain or spellbook. My favorite writeup is this post by Coins and Scrolls, which is incidentally also very in tune with my philosophy as regards magic-related worldbuilding.
The other old standby is Magic Words, which I think I first encountered via Papers and Pencils here. (There are more fleshed-out versions elsewhere, but unfortunately not any that have stuck in my long-term memory). This one posits that wizards learn spell-fragments, in the form of words like "fire," "missile," or "door," which they can then piece together to write their own spells.
I like these frameworks well enough, but I've seen them both re-iterated several times, and I wish there was more variety, or maybe more willingness to create something weird and out-there.
I also enjoy coming up with this stuff for its own sake. So here are brief descriptions of 50 paradigms, most of which are at least loosely compatible with standard magic rules that assume wizards exist and are people who cast spells.If any of these paradigms catch your eye, then maybe think for a moment about how you'd expand on them, or what the implications might be for a setting that uses them.
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Spells are...
1. Supplementary lobes grown in the brain using meditation and mnemonic techniques.
2. Fragments of the stolen mind of God.
3. Formulae describing a manner in which the delicate and volatile balance of cosmic forces can be safely manipulated.
4. Impossible sleights of hand invented by the King of Thieves.
5. Sorcerous bloodlines unlocked by convincingly fabricating the presence of a deity or lesser spirit in one's family tree.
6. Astrological rituals that invite the influences of the seven elemental planets.
7. Three goblins in an invisibility cloak.
8. Proofs that natural law is false or self-contradictory, promulgated by demonic philosophers.
9. The cobbled-together scraps of lost extrauniversal sciences.
10. Echoes of the word that brought the universe forth from primordial darkness, bounced back and forth across the cosmos from the day of creation until they were finally caught and decanted into jars.
11. Commands issued to a system of menhirs and henges empowered by ancient infrastructural rituals.
12. The sacred mysteries of dead, dying, or forgotten gods.
13. Ab-logical insights arrived at by taking psychedelic drugs or staring for too long into the depths of the chaotic planes.
14. Writs that empower eligible persons to make wishes with specific predetermined wordings.
15. Good advice given by someone at least a hundred years old.
16. Keys to various seals and limiters that were placed on mortals to prevent apocalypse.
17. Cheat codes for the universe simulation.
18. Koans, the contemplation of which allows one to perceive and transcend the illusionary nature of reality.
19. Astral weapons designed to be implanted in a soul.
20. Hyperdimensional structures that snap back into conventional spacetime when cast.
21. The caster's own externalized thoughts.
22. Chemical poultices and powders with a bit of something extra mixed in.
23. The secret arts of smiths, clockmakers, masons, etc.
24. Services provided by residents of the goblin market in exchange for items that suit their peculiar and highly specialized tastes.
25. Talismans prepared according to a series of exacting instructions and charged with the caster's will.
26. Phrases a bit like "abracadabra!" or "open sesame!" which for some reason only stick in the minds of the initiated.
27. Bullets etched with sigils and words of power, which ascend to a higher plane of existence upon being fired.
28. The misfortunes that escaped from Pandora's box.
29. Techniques learnt by beating a monster at its own game. (Defeating a basilisk in a staring contest, a giant in an arm-wrestling match, etc).
30. Favors that Entropy bestows upon those who hasten the world's decay.
31. The forms of a proscribed martial art that unbalances the vital energies of its adherents.
32. Sluglike things made of animate ink which pour forth from a spellbook to do the caster's bidding.
33. Objects or entities sealed within an extradimensional space in the wizard's hat.
34. Bizarre, temperamental devices that require constant maintenance and recalibration.
35. Pharmaceutical cocktails that briefly grant the imbiber psychic powers.
36. Patterns of mana that behave like cellular automata. Magic missile is a glider.
37. Parapsychological mutations induced by exposure to mythos entities.
38. Miracles worked by the various saintly relics that the caster keeps about their person.
39. Applications of the force exerted upon reality by purely mental phenomena such as perception or intent.
40. Demiplanes smaller than the head of a pin which follow altered laws of physics.
41. Stakes won from a set of anthropomorphic personifications that the wizard gambles with in their dreams.
42. Thaumic technology smuggled from the sunless lands beneath the sea.
43. Things the wizard sees with the eye they sacrificed for forbidden knowledge.
44. Divine retribution that the wizard directs to their own ends by tricking the gods with meticulously chosen blasphemies.
45. Otherworldly communications encoded in the movements of birds, the patterns of fallen leaves, the letters of magazines.
46. Hypnotic patterns inserted into the collective unconscious.
47. Things left over from previous iterations of the universe.
48. Secrets that you can only understand by emulating the mental architecture of an octopus.
49. Bursts of chaotic magic that in their natural state could do absolutely anything, but which the caster circumscribes with probability manipulation.
50. Ritualized dialogues that will persuade some vast and inhuman being to grant the caster a boon if executed correctly.
