In no particular order:
Chinese Assault Rifle
Pancor Jackhammer
Perforator (unique weapon fom F3 DLC "The Pitt")
AMR
12.7 Submachine gun
Marksman Carbine
9mm Pistol (Browning HP)
Colt .45 Auto
Gauss Rifle
Laser RCW
I could go on... basically I'd like F4 to be more like F:NV than F3, with a much greater variety of weapons, as well as weapon mods and the ability to reload ammunition. Playing F:NV it seemed like the guy in charge of the weapons for that game actually knew something about guns, so we didn't just get the (solid but rather dull) limited selection of mostly generic guns seen in F3.
Something I'd like too see that has not been in a previous game (as far as I know)? Somethng like the Arwen 37, the H+K "CAWS", and maybe the Vector 45.