I honestly don't play instagib either, but support for it would be nice. dna made some very good points about the problems with a cross-class instagib mode that I didn't think about. I've changed my mind on the matter and am more for uni-class instagib. I also completely forgot about Doom.
Perhaps rather than an instagib weapon for Doom, why not make it chainsaw-only? Doom has always been about close encounters, and I'd think it would make the gameplay more interesting rather than instagib rockets.
:offtopic
As for the mention of adding TA-like game-types...
I'm not trying to knock Generations Arena or any of the work put into it, but I don't play Gen for its team modes. If I want a variety of Team DM/CTF game types, I'll load up Threewave. However, I am looking forward to Resurgance. That to me, will be the best team mode Gen will have. I'd love to see variations on that.