What totally would be nice is for once a full-length break-open shotgun. Video games seem to be unable getting over including a sawn-off, break-open dualbarrel and full-length pump-action, the only exception I've seen is Hitman.
It would be nice to make a meele attack for the plasma gun - if you press the fire button whilst in meele range and the plasma gun is in the venting sequence (when it's lifted in the air to cool down after shooting), Doomguy would press the hot muzzle against the monster's body.
Some kind of SMG wouldn't hurt, however, not in the typical movie scenario - dual wielding Micro UZIs with removed stock. What would probably fit fine - UZI Pro with the stock in the correct position.
The chaingun is such an odd weapon - a stockless gun with Gatling-style rotating barrels firing the same bullets as pistol. Sounds like a cross between a Gatling gun with an electric motor and a sub-macine gun. What about giving it some kind of sipmle shoulder mount/stock, increasing ROF and calling it a SMG?
For the lovers of huge, fast firing nightmares - a larger version which mounts on shoulder like BFG9000 and fires, say, 5.56x45mm NATO rounds?
While writing this all made me think of some crazy powerup/weapon... the whole thing is still very unclear to me, but...some Hell-power based device that, if used in low-health situations, turns you into some kind of an undead...like you're on 20% health, you activate the device, demons finish you off...only to see you get back to the foot, now kept half-alive by the hellish device. The difference in gameplay from that point on would be that the hellish power keeping you alive would slowly drain, and could only be refilled with killing Hell creatures with the same device. Also, at some point of this undead gameplay, the hellish power would start to slowly crunch on your mind, making you more and more similar to the Former Humans wandering around - limiting your equipment to less and less weapons, and finally making you lose the control of your body, i.e. another way of game over. The only way out of that would be some kind of similar device/artifact, or finishing the game before this happens. I know that would be pretty much impossible to get into doom, but that's just a result of my imagination running wild.