Some ideas for new weapons:
Freezergun (Predator 2):
The gun is a modified version of the flamethrower. When it hits the opponent, the opponent's speed slows down and he turns white. When he has been frozen enough, he cannot move and looses health slowly. This weapon would be great for units like APE suit.
I could have sworn I saw this in R2 beta. Have the player start with 3 and have them setup around the map. They should be visible.
PROJAM grenade launcher:
This gun has the distinction of having Smoke screen, Nepalm launcher, EMP bombs and regular grenades. It is simply quite awesome and deserves a come back.
.45 with laser sighting:
A hand gun from the first Terminator movie. It works by having a laser sighting mechanism that can be used for better aim.
A larger version of a knife. Billy had it in Predator 1. I always think the corporates need a unit called a "Commando" which would have a pistol, a machete and a minigun.