I should preface this by saying this is a thread for anyone to request weapons made by ANYONE, not just me. It will give modders an idea of what we as the community are looking for.

With my rail gun wrapping up soon I'd like to hear what everyone would like to see in game.

Here are a few of my suggestions:

• A proper assault rifle. Many people were not a fan of the vanilla Assault Rifle. I think it's a neat design and even followed it a bit when it came to my rail gun but people want a regular ol' assault rifle for their arsenal. This could come in the form of a moddable AK platform or an M4 platform of some variety. Hell even my ACR would work for this.
• More pistols! I'm playing a gunslinger right now and my pistol options are quite limited. I'd love to see more variety. There's .45 bullets already, make a 1911!
• New energy weapon types. I'd love to make use of the piles of fusion cells I have at my disposal for some cool new forms of energy weapons. Not just the standard laser and plasma stuff either.
• VSS - A favorite from STALKER and other games. I love this gun. I made a variant for my ZL Armaments mod in New Vegas but frankly it isn't my best work. I'd gladly re-do it.
• A 5mm SMG. I made a P90 that did this for Project Nevada and it seems like it would fill an even bigger role in FO4. I don't know how custom animations would work with the new GECK though. It could easily use another animation base though. This way 5mm ammo becomes actually useful for those of us that don't lug around a big heavy minigun.

I made a http://www.gamesas.com/topic/1569802-weapons-and-armor-youd-like-to-see-added-in-dlc/ on the general discussion boards about weapons/armors people would like to see in a DLC... Everyone suggested something different, but some of the most common requests:

5.56mm Pistol (That Gun, modeled after Deckard's gun in Blade Runner)

.45 Auto Pistol (ideally imo, something that can be modded with auto or semi-auto receivers like the 10mm)

Chinese Assault Rifle (or really any more "normal" looking assault rifle - most people are also clamoring for a better variety of automatic weapons to begin with)

Bozar

Anti-Materiel Rifle

Grenade Launcher

Less common, but worth mention: Laser/Plasma weapons based on the designs from the original Fallouts, including Pulse guns, Pump-action shotguns, and more throwing weapons like knives, spears, and rocks. My own, personal request: Pipe shotguns, that can be modded into pistol form.

One thing that I'm thinking, though, is how new weapons can fill in any roles the current weapons don't. Like, how could a .45 or 5.56mm Pistol stand out in gameplay compared to a 10mm? A pump-action shotgun could be a bridge between the spread and power of a double barrel and the fire rate of a combat shotgun, but then you'd probably have to make new animations for it and that'll be tricky. Stuff like that.

And just a random question for modders: How tricky would it be to implement laser sights, where we can see the dot appear on our target?

Never was a fan, but a high quality Dragunov is always wanted

And yeah, the ACR was a great one for F3, and it would be right at home with F4's weapon modding system.

Plasma Caster. Seriously there isn't enough (useful) heavy weapons and there isn't enough plasma and/or use for plasma carts. Plasma caster helps fill both roles rather nicely.

I'd like to see a proper assault rifle in there too - an M4 platform like Zealotlee said, though maybe even more general - an AR-15 platform. You could start with a semi-auto M16 (similar to the "Service Rifle" in FNV) and mod it up to a tricked-out M4 sort of thing. There's a lot of flexibility there of course; CQB versions with shorter barrels, marksman versions with longer ones, even crazy stuff like the http://metalgear.wikia.com/wiki/The_Patriot from Metal Gear Solid for a super cut-down pistol-size variant.

Another I've been thinking about for a while - since the first glimpse of their weapon-modding system in the E3 reveal - is a Beretta 92FS, with mods that add in some features of the 93R. The three round burst (or maybe just full-auto), the compensator, the folding buttstock... right on up to the ridiculous top-level mods having it look like the "http://www.imfdb.org/wiki/Beretta_93R_Auto_9#Beretta_93R_.22Auto_9.22" from RoboCop... because how could you not?

I'd like to see pump-action shotguns with the appropriate animations and behaviour... and I'd like the Combat Shotgun to be completely replaced, actually. A Hawk Semi-Auto like in FNV, a Saiga 12, an AA-12 like I did in that optional replacer for WMK, anything - just not that ridiculous Frankengun they've got in there. (Same goes for the similar-looking "Combat Rifle", really - or maybe it could be turned into a PPSh-41/PPS-43/etc.)

A proper anti-materiel rifle, a semi-auto sniper rifle, maybe an M2 Browning for a heavy weapon... I could go all day.

I have been vaguely considering trying to make at least some of these myself - I think I'd like to get my skills up in that area - but I do still want to finish that mod for Skyrim first, so I'll probably be beaten to the punch.