I think it should progress through not the seven circles, but different understandings and severity of Hell- so the top would be standard Christian hell, with flames and such, and would get progressively worse and worse as the characters approached closer to the source of the Devil. 

I also think there should be an element of survival, with some weapons not harming some enemies- such as requiring a boss's item(s) or something to continue (which gives incentive for the characters to do this, it can be shown in some way that they progress in power).