I'm not against Duke at all since he's awesome, but also the point would open to "Why not Strife guy? Why not Lo Wang? Why not that guy in Powerslave\Exhumed." So there's my two cents.
Also why not Kirt? Why not the security cyborg from Marathon? Why not the guy from Cyclones? Why not some dudes from the black crypt? Why not the guy from ZPC? Why not the guy from Ken's Labyrinth? Why not the guy from that game on the 3DO where you're on a haunted island and stuff? Why not the guy from corridor 7? Why not heroes from a ton of other obscure 2D FPSs?
Okay, back to seriousness. What mystifies is your mention of Corvus and the fighter/cleric/mage being present in Duke Nukem. I only ever saw the doomguy. I had no idea Hexen characters were in there too! Where are they??!?