I think Duke Nukem had something of an identity crisis as a result of its long development history which resulted in a game that could not decide if it wanted to be Retro ( akin to the 90s era shooters) or Modern ( COD4/Half life onwards era). I think the developers were to afraid to commit to either branch for fear of alienating fans. This was a big mistake.
Duke Nukem Forever should have gone full retro-no cover shooting, health pick ups, no weapon limit, no driving sections and limited puzzle solving sections. It should have taken its inspiration from the likes of Duke Nukem 3D with tough varied enemy designs and a relentless pace. In fact, I think this approach would leave you with a game that would strongly resemble Doom 2016 in gameplay but much earlier. Duke Nukem could have also done with more humor drawn from 80s actions films-Terminator, Commando etc or perhaps follow the style of the new Shadow warrior which revived a similar cheesy one note character with decent writing and dialogue.
I think a Duke Nukem with some of the above suggestions could have been a real success and offered an alternative to the po-faced gritty COD-style shooters which dominated the market back then. If it did succeed then I could see a surge in less serious or more comic based shooters which focus less on realism for a few year.