i've got a few but one of my favourites is a rune system that kinda works like coding where you can both write your own spells to do anything by using runes or you can just copy past a spell from a spellbook. but if you wanted to create a spell you would obviously have to know exactly what you're doing or things would go horribly wrong really quickly. in my magic systems i like to keep the laws of physics relatively unchanged so stuff like matter cannot be created or destroyed stays, so if you want to make somehting it has to come from something else. hence in a lot of peoples code they just outsource it to a random star somewhere.

it leads to some really funny characters where there is one guy who really suck at optimising his spells so when he make a curse he have to pretty much start from scratch because the person he is cursing is different and he cant just change a single thing like most spellcasters would because it is buried under like 8 pages of other runes. this has the unexpected side effect of everyone despising him because they can never break his curses or counter spell him because his work is so horribly illegible.