I haven't had the chance to give the UT2K4 Chaos mod a spin. But I would like to say to the Chaos Team that from the screenshots and work I've seen, you folks have done fantastic work.
One of my instant favorites from the screenshot page was the grappling hook.
I personally feel that a tool like that should become a default part of Onslaught.
Its just too useful.

Anywho, one of my favorite weapons of all time was the Hydra from Unreal 2.
Yeah, Unreal 2. Remember that? A lot of people disliked the game, but I thought it had some pretty fun weapons. The highly versatile Hydra grenade launcher is a weapon that should have made it over to 2K4 as well, I feel.
For those who never played Unreal 2, the Hydra was a multi-use grenade launcher. It had 4 to 6 different types of ammo, if I recall correctly. There was the normal explosive grenade, as well as a smoke, force and EMP grenade.
The types were:
EMP grenade - electromagnetic pulse - affected only circuitry and electronics
Smoke grenade - when it blew, it lay a thick layer of smoke all around
Incindiery grenade - FIRE! FIRE! FIRE! This grenade set anything too close when it exploded on fire
Normal Grenade - normal explosive
Shockwave Grenade - This did no harm at all. But when it blew, a shockwave of strong force would give anything nearby an awfully strong shove - great for pushing enemies right off a cliff
I forget the last type. I think it was a corrosive acid grenade. Can't remember. I need to revisit that game.
Its possible that a request like this has come up somewhere else before, but I have not checked, and I just want to speak my mind.
I do not have any experience in modifying UT2K4, so I am not totally sure how difficult it would be to implement that classic weapon.
The Hydra's EMP grenade would be unbeleivably kickass when facing tanks on foot. It wouldn't harm or destroy the tank, just disable it temporarily. EMP (Electromagnetic Pulse) would smack all the circuitry in the tank - the driver would go blind for bit, and also be unable to exit becuase the hatches are computer controlled as well.
And an EMP shot on a Hellbender wouldn't do much, since the driver sees through the windshield, but the Bender's weaponry would be taken offline for a moment. An EMP shot on a Manta would cause the Manta to lose control - you can't steer or control the vehicle. A shot on a Raptor (hard to do because its a grenade, not a rocket) would bring the little jet right back down to Earth. The little buggy with the energy ribbon cannon (forget it's name) would probably be the least harmed by the EMP
The other type of grenade, such as the force grenade would be useful too. You could probably capsive light vehicles with it, or super jump if used right. Resourceful players could use it to toss off enemy Mantas (they're the lightest - they'd just fly right up from the shockwave) or dislodge stuck vehicles.
The smoke grenade would be excellent when dealing with Raptors. Hide in the smoke and use your AVRIL. The Raptor cannot see you and would have no choice but to run. You cannot see the Raptor either, but your AVRIL's targetting reticle would allow you to smack it nevertheless.
Incindiery grenades would make great traps - use it to set a vehicle on fire, like a Manta, fly it at a group of enemies or an enemy base while keeping an eye on the vehicle's health and bail out at the last moment. The fire would cause the vehicle to blow up more violently, so when it blows, it'll take a out a lot of folk or do a great deal of damage to whatever is nearby.
Anywho, in short, the Hydra needs to be included in UT 2K4. I don't know if someone did it already, but it would be a worthy effort. Had I more time, I'd learn and try to do it myself.
Again, like I said, I don't know about anyone else who played Unreal 2, but by far that weapon (and the Bio Spider gun, which would also make for a cool addition) were loads of fun.
Come to think of it, you could probably use Unreal 2's bio spider gun to counterattach UT2K4's robotic spiders. The robotic spider mines focus only on you, so if you were able to get some bio spiders of your own, they'd attack the mines.
Anywho, just ideas. I would appreciate feedback.
Peace.