http://en.wikipedia.org/wiki/EM-2: It looks pretty futuristic, was developed in 1951, and was developed by one of our allies. Place it in 5.56mm rather than 7.62 though. Or bring in 7.62, and create a few weapons that use that round. I'm sure every gun nut would love to have the AK-47 if 7.62 is brought in.

http://en.wikipedia.org/wiki/Pancor_Jackhammer: Also pretty futuristic looking, but a blast from the past. This could be 12 gauge, fully automatic shotgun. Although powerful, it'd eat your ammo right up.

http://en.wikipedia.org/wiki/Smith_%26_Wesson_500: Sure, there's the 12.7mm pistol and submachine gun, but neither of those have the grace and beauty of a revolver. This weapon would hold five rounds, and would be chambered in 12.7mm/.50 AE.

http://en.wikipedia.org/wiki/Colt_revolving_rifle: As you can probably tell, I love revolvers. Why not a revolver rifle? This would be chambered in .44 Magnum, and would house six bullets per clip.

http://media.moddb.com/images/mods/1/4/3047/24758.jpg: Seeing as .50 BMG is only chambered for the Anti-Materiel Rifle, what better way to expand it then to bring in the gun that bullet was developed for? Seeing as the M2 is pretty darn heavy, and not meant to be used as a moving weapon (it's meant to be stationary, even if it's mounted to a humvee, it's still stationary), this would have to be modified a bit. Also I suggest it has a somewhat slower firing rate, but a huge amount of kickback. This weapon although powerful would be pretty darn inaccurate.

http://world.guns.ru/sniper/sniper-rifles/rus/vss-e.html: Although the war was with China, and not Russia, the Chinese were notorious for copying weapon ideas. The Chinese Type 56, which the Chinese Assault Rifle is based off of was a Chinese
clone of the AK-47. It wouldn't surprise me if in the Fallout world they too copied the VSS Vintorez, a silenced fully automatic 9mm sniper rifle that was designed so it could fit into a briefcase.

http://world.guns.ru/sniper/sniper-rifles/rus/sv99-e.html: Seeing as the Varmint Rifle uses 5.56mm, I thought .22LR deserved a rifle. The SV99 is yet another Russian rifle, but could be copied from the Chinese. This gun is designed to take out lights silently, but at times is used as an anti-personnel rifle (in case you didn't know, a .22 to the skull has enough force to break through the skull, but not enough to escape it. Because of this, the bullet simply ricochets around the skull, tearing apart the brain until it's stopped dead in it's tracks).

http://www.fullyautomaticairgun.com/: So there's the Red Ryder BB rifle, but why not a BB machine gun? Not quite as strong as the American 180/Silenced .22 machine gun, but with a faster firing rate.

http://en.wikipedia.org/wiki/Mosin-Nagant: My personal favorite rifle, I suggest the Mosin-Nagant be brought in if 7.62mm is brought in for the EM-2 or another similar weapon [AK-47 anyone?].

http://www.seahornet.com/spear/spearguns/marxmangun.htm: While not a gun in the traditional sense, the Spear Gun is essentially crossbow that fires an oversized arrow. Either new spears could be implemented as ammo, or if possible, this gun could be modified so it shoots out throwing spears instead. Though I'm sure that would be problematic.

Molotov Cocktail:
Ratios of each item will vary, as I'm sure there will need to be more than just one of each (except the empty bottle) for balance reasons. The Molotov Cocktail is similar to the incendiary grenade, but with a bigger blast radius, but less damage.
-Turpentine
-Empty bottle
-Box of Detergent

Pipe Bomb:
Stronger than a stick of dynamite, but with a heavier weight, the pipe bomb is a prospector's new best friend.
-Lead Pipe
-Dynamite
-Duct Tape (?)

http://fallout.wikia.com/wiki/Bio-gas_canister:
A weapon from Fallout 3's Point Lookout, I thought the Bio-Gas Canister was creative and interesting. Lower the damage from 1 to 0, and make them spawn randomly, rather than a short supply like there was in Point Lookout.

Flashbang Grenade:
Vipers got the drop on you? Jackals kicking your butt? The Flashbang will save your day by leaving your enemies disoriented and confused. But watch out, it can work on you too. (the flashbang would do very little damage, but would have the same effect as it does in Call of Duty and other games with flashbangs, where the screen goes white, and a buzzing sound is heard).

Nail Grenade:
Frag grenades not enough? Just glue some nails on and let the fun ensue!
-1 Frag Grenade
-? Wonderglue
-? Nails/Box of nails (new misc. item)

Mini-Nuke Mine:
Add the power of a Mini-Nuke to the tactics of a mine!
-1 Mini-Nuke
-1 Lunchbox
-? Cherry Bombs
-? Sensor Modules

Nerve Grenade:
Similar to the Bio-Gas Canister, however it deals damage over time for anything within it's hazy, yellow cloud.

Knife Bomb:
While the knife is an effective weapon, one good shank doesn't always finish the job. With the knife bomb however, you're sure to give any enemies one hell of a blast!
(the knife bomb could either be craftable, or a new weapon that randomly spawns. I suggest it works by getting close to the enemy, placing in a thrusting animation, and then having the knife stick in them. Then the player has about two or three seconds to back up before the enemy goes kaboom)

Dynamite Spear:
A spear's always great, but an exploding spear is always better.
-1 Throwing Spear
-4 Sticks of Dynamite

Proxy Bomb:
The poor man's C4.
(essentially a new version of the powder charge that works by using a detonator rather than using a sensor module and working like a landmine)
1 Tin Can
3 Dynamite
1 Scrap Electronics
1 Duct Tape

http://www.free-images.org.uk/military/07-flare-pistol.htm
Need a light for the darkness, or an enemy set ablaze?
Flares would set enemies on fire and do damage over time, but would be weaker than say, the Incinerator.

Flares:
Similar to the Flare Gun, but thrown. Flares do hardly any damage, but create a bright, radiant crimson glow for those dark corridors.

Pulse Rifle: There was the Pulse Gun, which is a pulse pistol, but why not a pulse rifle? Super effective against robots and power armor, but weak against human targets. I suggest using the http://fallout.wikia.com/wiki/YK42B_pulse_rifle_(Fallout_2), but change the diamond pattern of the coils by the base of the gun with an X pattern to allow an ADS design, or a scope to be mounted.

The Boom! Box: A much different weapon, the Boom! Box is a bulky sonic rifle. Using the same animation as a Mirelurk's sonic cry, the Boom! Box can also easily cripple enemy heads, but unlike Mirelurks, it's not very devestating. It will however make any enemy stop their firing to try to get the ringing out of their ears.

PEP (Pulsed Energy Projectile): Similar to the Plasma Rifle, the PEP fires an orange glob of energy (instead of the Plasma's green goo) that on impact sets any targets ablaze.

High Intensity Radiation Laser (HIRL): Ever play Halo 3? You remember the Spartan Laser? Basically a clone of that. Essentially a laser-based Tesla Cannon, that also puts out the radiation level of a destroyed mini-nuke. Highly effective against non-ghoul organic targets, but not so effective on robots.

Tesla Trident: A special, three-pronged staff with electrical shockwaves jolt through the three-prongs. Towards the opposite end are two slender batteries designed to counteract the weight of the electrical head.

http://en.wikipedia.org/wiki/Fire_lance: While the Fallout version would be more modernized, this is what it's based off of. The Fire Lance would be a large metal staff that at one end has a bit of gunpowder and sharpnel. On impact, the head would discharge a small explosion similar to the Ballistic Fist.

http://en.wikipedia.org/wiki/Pilum: A Roman spear, the Pilum was designed for both close quarter and thrown combat. Once it impaled an enemy, it was designed to break so the enemy could not use the spears from fallen allies against the Romans.

http://en.wikipedia.org/wiki/Morning_star_(weapon): Well if Power Armor is supposed to make you look similar to a knight, why not give him a medieval weapon? =P This could be a more modernized version of a broke table leg with black nails jutting through the broken head of the leg.

Defibrillator: Need to jumpstart a heart or stop one? The defib is for your best friend. The defib would be a two handed melee weapon, where both fists are pulled up, holding out the paddles, and the attack animation would the character thrusting out the paddles. On impact, some static would bounce around the enemy and knock them back, possibly knocking them out (based on your unarmed skill level and luck).

Katars: For anyone unfamiliar with katars, they're an Indian (that's eastern, not western/native American) weapon that's essentially a wooden handpiece attached to a large blade in front of the knuckles. It's generally used as a stabbing weapon, but also is designed as a slashing weapon.

http://en.wikipedia.org/wiki/Tonfa: Not a night stick, the Tonfa is a large beam with a handle. It is wielded by holding the outer piece, and having the long beam extend down the forearm for blocks and punches (protects the forearms, and the head of the beam increases the force of a punch), or with the beams sticking away from the forearms and pointing out from the knuckles for increased range and blunt attacks.