I'd say a Mad Max world would start with us in a settlement, then proceed with us moving out in a war rig or other small vehicle fleet. Perhaps have it mostly play out like Fury Road (since that was the one movie of the series I enjoyed the most).

We wouldn't need to be familiar with Midgar as a whole - even in-game, we never visit more than two or three sectors, a couple of the Mako Reactors, and the Shinra Building. So long as we have an idea of that, we're good.

No Mercy or Dead Center would be pretty good scenarios for a zombie level, I think. Straightforward, while still being hectic.

Going to the worlds where our characters come from? While I guess my iteration of that should be Fortuna / Fortuna Castle, I could also go for the Temen-ni-gru from DMC3. Although both would probably just be damn confusing at times.

I say if we go for a Hell level, make it from Doom. It'll be the ultimate bloodbath of hellspawn.