Ok, there's 36 pages of posts. I haven't read all of them, so this may have been posted elsewhere.
After finishing a certain amount of Arch Angel missions (say 3) with an Arch Angel of Humanity as a definate, or at least once you you've finished all the Angel of missions on a decent difficulty, open up a new menu of challenge missions. *Foo* of Hell missions. Or perhaps just Single one, "Daemon of Mars", in which you have a class for each type of bad guy. Where you take on the persona of each of the game's enemies, with it's own mini-skill-tree (just normals and advanced), or a generic standard skill-up for levelling with mini-skills (or mods for standard equipment). You could take on hell at it's own game. A random amount of boosted formers spread throughout the levels (quite of few of them, with 1-8 traits, 50-70 health, modded equipment and armour) to represent all the marines that had a chance and died before Doomguy came along.
For some ideas on the daemon types, give them all their normal HP+5 with strange weapons or accuracy/speed/damage systems. Some weapons can run on "mana" (a constantly regenning ammo supply), hate (how many kills over the last "x" seconds), rage (H2H damage in the last "x" secs or turns, either from you or for everyone), fear (damage taken), trickery (boost for attack or ammo from dodges recently), scavenging (takes time, but you feed on corpses), daemon-tech (consume mod packs for ammo/damage boost) or unlife (health globes/super charges/invulnerability/berserks are your life and your damage/speed/skills). You could combine these or make them do slightly different things for different daemons (a skull gets a brief non-red berserk style boost for dodging, while a daemon gets its damage boost from taking damage), and life depending on their own form of carnage (skulls lose life whilst running and waiting, are naturally fast but have little normal dodging whilst daemons regen life but do little damage until they take some themselves). Having the more tech-based daemon with regenning armour and mod-packs given on level up? Or imps that never run out of ammo but only do some real damage when they grab some life? A pain elemental that gains life off the damage their (or any) skulls do, but regen ammo (skulls) from damage taken? Mini-whizzkid-levelling, tech-eating arachnotrons with plasma guns they fuel off corpses? Why not? Pretty much an entirely new game, with a nicely evil slant to it. With the right traits, level-up types and boosters/combat styles I think it would work nicely. There's a million ways you could do it, to hell with perfectly balanced (yes, technicians are the best).
Perhaps with a yet another challenge level beyond this. Archdaemon of Earth. What this should be, I don't know....