Now that the floors of hell will be deeper'n'eva on a harder difficulty, why not more level types to enjoy on the way down (and limiting most type if played on a easier level).
Now I don't remember much of doom myself, because it's been a while so some support of some ideas will really be helpful(and I'm cheating by looking at a doom strategy guide).
Hell's jail type - Much I mention on the previous forum, it's not really a literal jail(from hell, heh) but monsters are in small rooms behind bars, shooting at you (which the impassible bars can be shown as a colon).
Chapel type - okey, my brain died at this point and I don't really know if their is such a thing. But it's like a special level but the level is generated in a structured building. (Oh god, it's a bad idea already). But I was thinking about those nice dead trees I be seeing in the final part of doom. Maybe just trees about and single room buildings about.
Void - this is my own. Blank spaces are bottomless void. To fall off into the void is by force (and not just walling into it). So wielding a shotgun is recommended for quick work on grounded monsters. Be prepared to run to solid ground when you see cacodemons floating over it and you are in a two space wide bridge-like platform }=).
That's all from me for now.