As for mad max with wizards: well, what is typical for mad max, and postapocalyptic settings in general?
Scarce resources and societal collapse seem to be a staple. Makes society more brutal as people fight for resources, some band together, some start raiding to survive, all that good stuff. But what to make scarce in a society with magic?
Why, perhaps the 'resource' that allows magic to be done? Imagine it, wizards having to scrounge the land for the precious few remaining sources that fuels their magic (lore wise, explains why society collapsed: a golden age after magic was discovered/learned, but decline as the source became scarce). Gives a nice gameplay mechanic: you can use your magic as you want, but if you use too much without securing/finding a way to refuel, you're screwed. Encourages greed and backstabbing.
Oh, and maybe make it so wizards who go without this 'fuel' for too long start loosing their mind and going insane, maybe even start mutating into horrible monsters. So embracing magic gives enormous potential and might, but like a nasty drug it means that once you get hooked, you're in it for life, one way or another, no turning back. Always looking for that next hit. And gives even more urgency for getting that magic fuel. Maybe the monsters hunt wizards to feed that primal urge for the drug, cause they can't do the ritual to harvest the fuel into a usable form, so they just eat those who have some in them.
What the fuel exactly is, could do several things. Something you ingest or inject, or which you tap from natural sources. Maybe some oil-like substance which flows from certain places. Or make it so 'life force' fuels magic. Meaning you have people who carefully tend to gardens or lifestock to harvest their life force, while others hunt around for natural sources of it, while the most ruthless hunt humans or other wizards (cause those are the most abundant sources of lifeforce).