There is so many sources of magic, it's like every author inveted one

Well I think magic need to be balanced to prevent mages from taking the world over. That is why I like the magic system in The Forgotten Realms. As I recall, it's like a web where magic flow. When someone uses magic, it drains the energy and slowly regenerate. We never appllied it I think but if you overuse magic, the ressource will be depleted. It is supposed to regenerate over time but not if it's completly destroyed. It's like a natural ressource. When are the fishes are gone, they can't regenerate anymore. But in Faerun it's nearly impossible ...

The other one is when someone need to do some sort of sacrifice to perform magic. It could be gore, requiring the sacrifice of livings or part of the body... or simply that the mage get some exhaustion after each spells. Both mental and physical.