I've been kicking an idea around lately, and since I don't have the expertise to do it myself, I thought I'd just share it in case anyone else wanted to give it a shot. Also, it may already exist, and if so please lemme know.
I was imagining a new enemy consisting of a redesigned Doomguy model meant to be the Nega-Doomguy, if you will, in a fight sort of like the doppelganger fight in Symphony Of The Night or the shadow fight in Zelda II. He could be given the moving frequency of the Arch-Vile with cranked up speed, and his attack could be a re-coloured Arachnotron plasma shot (coloured blue/white to resemble the Plasma Rifle). Or he could just be built mostly from the base of the Arch-Vile and just recolour his flame attack green to resemble a BFG blast.
I'm sure it'd be quite a task to create, but the idea intrigues me.