I have already started a Doom mod for halo, I've already got all the sounds in, which took well over two months. (I don't have all the time in the world to do things on the pc) And, I'm getting ready to convert a whole bunch of things. The pistol was converted into a Jet Black Winchester Handgun, the Warthog is a dune buggy, and the tank, looks like an WWII tank. The flamethrower was converted into a chainsaw, and it's the perfect choice, because it's programmed with an "idle" sound, so I replaced the flame idle with an chainsaw idle. Anyways, it's coming around nicely, so expect a beta to come out soon.
FINISHING UP:
Assault Rifle -> Gatling Gun
All plasma guns -> BFG
Shotgun -> New Doom-Style Model
New HealthPack Model
New Ghost Model
Whole Bunch of Texture Work
New Rocket Launcher and Rocket Projectile models
Updates soon.