If I could add the things I knew were supposed to be in DNF, or what I was expecting, here's my list:
-Wide open levels with no rails, based upon actual blue prints of real places.
-A huge Las Vegas hub where you could enter and explore every single building, enter buildings in any order you choose, enter a few buildings, or not enter any buildings at all. Exploration of the hub would be key to finding secrets to benefit Duke, or expand upon the story.
-The choice to continue the game with side characters, or not if you killed them.
-Once Bombshell is your ally, if you try to grope her bosom, or, heaven forbid, shoot her bosom, she takes out a super shotgun and gibs you.
-Constant dialogue battles between Duke and Bombshell, where Duke tries to flirt with her, and slams him down. The game should end with her still not being attracted to Duke, despite saving the world with him.
-Return of some essential Cycloid enemies from Duke 3D
-Bring in the cut enemies, such as the Ant Guards, those Hitmen which Duke sniped in the 2001 trailer, the terminator bots, Morningwood vultures, the infested EDF guards, and the Grey Aliens.
-Return of Proton's Techbots from Duke 1, which are larger, deadlier, have more maneuverability, and clever A.I. Like the Foot Techbot.
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-DNF's concept claimed that Dr. Proton summoned a whole array of alien species through Area 51's communicators, so bring in some new alien enemies as well as old ones. The Rigelatins should return again, so I think that perhaps the Rigelatin slug guys should appear dressed up as Boy Scout Masters on mechanical horseback? Then maybe a selection of the classic Rigelatin enemies...
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-An abundance of interactivity with the NPCs and the environment.
-Duke's character having a little bit more development, and some good dialogue and interaction with the NPCs.
-Original one-liners, without abusing their "homage" to Evil Dead to death.
-No repeats of the original Duke 3D bosses... Okay, perhaps as flashback battles, but I wanted the DNF bosses to be completely new, enormous, challenging, and fun.
-A flashback battle with Dr. Proton in the deep, deep, deep Jetsons-like future, where Duke recalls seeing him crash into the spiked floor and explode in blood and nuclear confetti.
-Those spaceships from DNF 98 trailer.
-The motion and blood effects from DNF 2001.
-An enormous boss battle at a military airport, where the titan alien crashes through the airports window wall.
-The solar flare effects from DNF 98.
-The strippers, hookers, prostitutes, tourist women, and singers all having a little more personality.
-Expanding upon The Thing concept from the DNF 2001 trailer.
-Boss battles with the giant robot and giant Proton screen from the DNF 98 screenshots.
-The red chainsaw.
-Three different boss encounters with Proton.
-A well done twist at the end.
-A highly satisfying and original boss battle, or at least one that's very much fun.
-Gus attacking enemies with his pick axe.
-An cathartic climax, and a satisfying, multi choice, interactive resolution ending.
If done today as a mod, I would opt for either Knightmare's KMQuake II engine, or the Unreal Tournament 98 engine.