@HelloProject I actually had a project like that I never got to finish. But I'll describe the concept if you want a slice of any of the material.
You have Hell, with Seven Patrons, based on the Seven Deadly Sins (Reference a guy named Peter Binsfeld, he's a Catholic witch hunter and theologian, Wikipedia has the basic list of the demons, you can do deeper research from there). The Seven Patrons are NPCs controlled by staff, and the politics of the MUSH is about the Seven Patrons vying over Hell, with each PC falling via a Mortal Sin that makes them beholden to one of each Patron. The Mortal Sin is listed on the +finger, as important as Quote and Profile.
You have them start as Sinners, and if they advance (through however you want to do it), they can become Demons. Alternately, you can have them attempt to Atone for their Mortal Sin (by however mechanism you deem fit), and escape Hell.
The key would be the goal of politics. You could possibly have them influence the Mortal World, master various forms of magick, court the older demons for extra power (an artifice system, perhaps, where Pledging yourself to a Demon with a higher rank granted both of you power), or maybe even the old fashion armies of Hell battling back and forth.