Cool idea. I'm having trouble thinking of a traditional Final Fantasy job that would fit the bill, but they could invent something new. Maybe a Warlock or Necromancer style of class. There's a lot that could be done with heals that are derived from draining health from enemies, or even other allies. Auto-Revive could be a thing. Or a "Blessing of Undeath" buff that lets an ally continue to stand and fight for a limited time even after they reach zero HP. Sap the strength of enemies as a way to mitigate damage.

I know we already have an "Alchemist" as a crafting job, but we could have a flavour of combat healer fashioned after that as well. If you've seen Full Metal Alchemist, you know how dark that show can get. Lots of potential to tap there.