I had an idea for a setting last night.
In a world where common sense is that running out of MP means becoming severely weakened, passing out, or even death if you want to get that serious, a person is given a magic class with the kingdom hearts rule, more or less.
MP will not naturally recover until it is exhausted, upon which it will enter a state of accelerated recovery, but it needs to fully recharge before it can be used again. Potions, items, and sleep will still recover MP, but otherwise it will not recover on its own.
Basically anyone who has MP has at least some automatic/natural recovery of it, even if the speed and amount differs.
Whether the main character is aware of how this fully works to begin with, or they have to discover it by accident, or through experimentation... well, I think having to discover it is more interesting. For added difficulty, maybe even remove the ability to have it recover via sleep, and have it be only potions, and items.
Oh, and it has to be consumable items. Equipment that improves/provides MP regen will have no immediately observable effect, but will speed up the accelerated recovery.