I will say this with the preconcieved concept that the game will have two firing buttons, fire and alt-fire, to expand upon the functionality of whatever you're holding.

1. Melee level weaponry
Surely you don't want to be forced to take on an army from Hell with just your bare hands. If nothing else the standard equipment M9 bayonet works as a dandy stabbing utensil. Normal use would imply quick stabs, while alt-firing would toss out delayed but powerful lashes into your opponent. Improvisation would work well in this department, having your character resorting to rusty lengths of pipe as bludgeoning instruments or just tossing a nearby chair to delay the inevitable. It'd be nice if the chainsaw would make a return, neverminding the fact that a chainsaw has absolutely no place in a UAC facility on Mars (Unless they had a dome with vegetation in it, but that's just stretching the point).

2. Small Caliber weaponry
The standard pistol caliber of today seems to be 9mm, although 10mm is much more suited for military use. In any event your character should come equipped with a pistol from the get go. Taking a lesson from the G18 and the M93R of today weapon versatility is everything. Therefore, I will hypothesize that a pistol of the future would resemble a Glock in both simple design and having as few moving parts as possible. The weapon would have no safety switches but rely on the firing mechanism to not accidentally discharge. It would definitely have scope mounts and perhaps even a laser sight built in even though soldiers use 'natural point of sight' shooting and never need to aim in a gunfight, but these would remain handy for pinpoint shooting requirments (such as taking out a bomb from a safe distance). Going with 10mm ammo I can picture this large handgun holding a full 15 to 21 rounds in the chamber, all depending on design. There would be a selective fire switch on the sides, probably ambidextrous, that would select between semi-auto, burst, and full-auto functions. Semi-auto is self explanitory, one shot every time you pull the trigger. Burst lets out three bullets every time you pull the trigger, to make sure who you hit goes down. Full-auto would be reserved for very close quarters combat where ending the gunfight as soon as possible would be best. Sure makes respawning easier to handle, though.

Any upgrades available for the small caliber weapons could include finding another pistol and using them simultaneously. This could be very handy early on into the game, targeting two enemies at once. Another possible upgrade would be to a submachinegun weapon, and the best one of these I could base any reference to would be the MP5A5. Small, compact, highly customizable and functional. Quake 2 had one of these so I know it's still in iD's blood.

3. 12 Gauge weaponry
Although pump-action shotguns have been made obsolete by the Benelli M3 Super 90, video game style still dictates that these remain. Instead I would like to see the Turok 2 approach and have a pump action double barrelled shotgun as the default. For once I'd like to see an FPS that has the spent shells ejecting from, gasp, the ejection port and not flying right out of the barrel (If they could do it in Resident Evil they can do it in Doom 3). The secondary fire mode should be single barrel shots, just for those occaisions when ammo is scarce and you can't afford to waste any.

A perfect upgrade to this would be a double barrelled incarnation of the AS12. Picture a shotgun in your head. Add another barrel. Remove the magazines underneath and replace them with a drum just ahead of the trigger. 24 rounds a drum sure beats 8 in a magazine any day. The big advantage to this would be that the weapon is semi-automatic (no pumping required) so the shots could come out faster, as well as shorter reloads (and a longer time between reloads).

4. 5.56 NATO weaponry
I find it doubtful to see that this cartridge would still be in use over 700 years from now, but nonetheless these weapons should utilize high powered rifle rounds. First up would be a standard assault rifle, designed however you think would fit. Taking an M4A1 and 'futurizing' the hell out of it works fine for me. Normal fire should be full auto that rips about as fast as the original chaingun in Doom. Secondary fire would be three round bursts to conserve ammo, although the three rounds would be shot faster this way. I can easily see this weapon using 30 round clips and being reloaded very fast.

Next up would be the old iD standby, the chaingun. There's quite a difference between a chaingun, a gattling gun, and a minigun but all we can hope is that this weapon is done better than it previously has. Spin up times should be obsolete by this time but spin down times are inevitable. Firing should be balls to the wall, all six barrels blazing flaming death. Alternate firing should only utilize the top barrel (at a speed still higher than the assault rifle) to make the ammo last six times as long. Please, no attached grenade launchers or ultrafake 'extra fast!' firing modes, they've been done to death.

5. 40mm Grenade weapons
Quake 2 was ingenius is this respect. Futuristic grenades that can function just as well when thrown or launched. Perfect, as I see it. The first weapon would be when you just find the grenades without having a launcher handy. Firing them should show your character pressing a switch (or whatever) on the side and heaving them, giving a five second delay before detonation. Bounce one off of an imp's head and it won't be able to get back up before it goes off. Alternate firing should press a different switch (or whatever) and have them go off on-contact with anything they touch, enemy or wall. Much more dangerous this way, but also more flexible depending on the situation.

The upgrade would naturally be a drum-fed grenade launcher that can shoot them farther than you could throw them, and a faster rate-of-fire as well. The operation should remain the same, firing gives 5 second delay and alt-fire gives on-contact operation. Of course, now you can plant a grenade inside of an imp and make it a walking time bomb. :)

6. 30mm rocket weaponry
I am quite dissatisfied with all of iD's rocket launchers up to this point. They never need reloading, the rockets (given their size) fly far too slowly to maintain momentum and the explosions are far too small. All of these problems can be easily fixed if you just change a few simple design flaws. The weapon should be the size of the one in Q3A but designed drastically different. Instead of the barrel being so damn large I see it smaller, but a little longer. There would be an excess under it to house the electronics. These small rockets would fit five to a clip, and given their size you can easily justify their slow flight and explosion size. The only alt-fire I could imagine would take a cue from Soldier Of Fortune and release a quick cloud of death, all five rockets as fast as possible. You should also be limited to 25 or fewer of these (else you risk randomly exploding when attacked).

7. .44 Magnum weaponry
Nothing gets to the point like a metal slug that can punch right through a truck engine. One of these shots should cause as much damage as a shotgun blast but gain power the further you are from the enemy. Up close a .44 mag would just plow right through you but at a distance it would leave a tremendous hole. I doubt that iD will implement this but personally I would love it. If it is in there the ammo should be ultra-rare just to making using it an extra joy.

8. Energy Cell weaponry
As nice as the Plasma Rifle is, I think it should remain the better of these two. The first one you come across would be smaller and less functional, only having the primary firing mode that doesn't go as fast as it did in Doom. Once you obtain the full fledged Plasma Rifle though, the rate of fire would return and add a new secondary attack to try out. The secondary attack I picture would hit instanty (no travel time), do twice as much damage per hit, and eat up 4 times as much ammo just to keep you from doing it too often.

9. The BFG
It should be a lot more functional. Normal fire would be the Q2 style, alt-fire would be a spherical shockwave around the user that doesn't do as much damage as the other mode but is more defensive in nature. You should be able to control how many cells it eats by how long you hold down the firing button. Giving only 20 cells would not make the weapon nearly as effective as feeding a full 50. Feeding the defensive sphere a full 50 should make it a suicide weapon, going off in a humongous meltdown that instantly liquidates anything within the sphere (naturally this would be effective only in online multiplay).