I'd make it an open world game where you can find a hidden jet pack in a window on the ground floor of a sky scraper, and spend 10 minutes flying up to the top of the tallest building in the game where youll fight a giant tentacle boss monster that spawns waves of monsters in eggs and you have to shoot it's various weak points.
Through out, there are women In the battle area, and they die. And as they die, Duke gets madder and madder because it's less pussy he gets to crush, if you beat it with all the girls protected you get a special ending, and you also get a special ending if you do the jet pack thing. (Something like "fuck all this bullshit, I'm going after the top dog."
The rest of the game is corridors, hidden rooms with hints at the bosses weak points and DOUBLE DAMAGE zones, where it says "DOUBLE DAMAGE", and if you find them and hit them, duke says "DOUBLE DAMAGE" like a fuck head every now and then.
I'd have key cards, and colored doors, and secret rooms with armor and shit, and when you equip it, it changes dukes appearance, and when it gets weak he rips it off and throws it to the ground and makes a quip about armor being for girls or some chastity belt joke or something.
I'd make 20 guns, and you can lug around all 20 guns, and when you get the last one you have duke go "Hell, what's one more?"
Talking shrink ray, ice beam, multi launcher, mini BRRRRRT, sniper rifle that shoots through walls and leaves holes in the walls it goes through. Trip mines, floating enemy seeking mines that are obnoxious pussy seeking fuck bots duke reprograms for murder.
I like the idea of a gun that just changes the monsters into duke, and when they see the real duke and they know they can't compete with The Legend, they kill themselves.