As you can tell in Kaiser's story, the Doom marine stays in Hell for 7 years. This is what I want to focus on. Now, fitting the Tech style in there will be easy, thanks to an idea from your original story: Demons creating structures like those the humans have, yet twisted in a sick mockery of their former glory (this part I really like). There will be 3 'styles' if you will. Hell, Tech, and Dark, in that order. Hell is self explanitory. Im making hell the easier part because since the motherdemon is gone, her closest minions are also destroyed/weakened. Tech will be like doom64's tech, except that there will be unearthly hues to the walls, and ambient sounds aplenty. Dark is much like the 'blue-ish' levels of doom64. Its like limbo, where you'll do alot of exploring/trap dodging, but do fight monsters too, expecially in the later Dark levels. Anyways, back to describing story. So 'you' are in hell, moving from area to area just trying to rid hell of demon threat (threat = larger demons mostly, its not like there are consiquences for missing monster kills in a level). Anyways, once you reach limbo, you see that when people are dying, their souls are going straight to hell, rather than being judged in limbo. This is because the demons have erected a large technology oriented Spire (created using the same original teleporter technology, just twisted beyond human recognition) which pulls in ALL souls to Hell's Maw (the name of the final level...i hope nothing else has used that name, but I have a sickening feeling that its in Final Doom :/. So if becomes clear that you have to destroy the Dark Nexus (aka the Spire) to prevent hell from unfairly adding to its ranks.