MY ideas for weapons (how to implement in 3D):
Blaster: the blaster should have a rechargable battery (gains energy from air), but shooting a lot will be slowly draining it. Wait a while if the battery drains.
BBT Launcher AKA Bouncer: This weapon should be rapid-fire. Of course it would be good if bullets were bouncing randomly.
Frostbiter: Like in DN3D, it would be cool if Jazz could smash frozen enemies into pieces... Also, something probably hard to do - to make frostbiter freeze water in the environment.
Seeker: The seeker would be much better if it didn't hunt down all enemies around, but the player had to lock on the target and then fire a rocket that will seek it.
RF rockets: This idea I took from UT games - depending how long you keep the fire key down, the more rockets you'll shoot (up to 3).
Toaster: Just a flame thrower, what more could be said? Think of the one in RTCW for example. Or JJA, where enemies are running around burning. Robots would be immune to toaster of course...
TNT: The player would have to hold the fire key to adjust the timer and then unpress it so he will leave the TNT.
Pepperspray: This weapon would be something like shotgun from Quake games or Flak Cannon from Unreal games. Just come close and make swiss cheese out of your enemy.
Electroblaster: This weapon would cause electrifying enemies, also causing a chain reaction when there are few enemies close to each other. It would have immiediate impact and rapid-fire (continous, like toaster). But some enemies would be immune to electroblaster with help of special suits.
Slimer: This weapon would blast acidious slime, that would slow down enemies and cause them to slowly burn. It would be also capable of setting sticky traps, weaker than TNT, but more useful.
UWoCD (Ultimate Weapon of Certain Doom): A big rocket launcher that is very heavy and rare. One shot with a quite slow rocket, and we have a nuclear explosion.