I got an idea like this, for one character! It was for a crossover game...I wanted to have Spawn join the party. He would start with 9999 MP, but have no way to restore them. Now, I really only know a little about Spawn, but I know that he has Magic that consumes his 9:9:9:9 supply permanently. He can fight with many other ways, like his sentient cape, and I think he's skilled with guns. He probably got some crazy stuff that lets him ignore that rule, I don't know!
So from that, I got an idea for my own character to work like that. The main character would have obscenely powerful magic that consumes MP permanently, and running out would end the game. But they'd have a special gun too, that uses ammo. But maybe ammo is limited too. One other character has skills that permanently lower their max HP. It was to be a survival type of game...so it works. The idea for the main character was like Spawn's 9:9:9:9 magic + Breath of Fire Dragon Quarter, or something, while that other character was inspired by MMBN4's Dark chips.
I happen to love different takes on MP, and other things, so I say, give it a try/chance.