As you may or may not know there was a thing called "Map Themes" in old days of minecraft, and one of those themes were called "Hell".
All of the other themes make their way into the release one way or another, however probably because of consideration of nether at the time they removed the hell theme.
Hell had few unique things like different sky and cloud colors, no grass, lava instead of water and no natural light just like deep dark from the mod Extra Utilities.
However it was still a functioning world with trees and overworld mobs (however due to the no grass aspect it may didn't had friendly mobs, which is a thing that should be changed if this idea becomes a real mod by allowing mobs spawn on dirt too).
It might can even have the nether as haven or an ultrahell