I don't remember the original DOOM including several playable Resident Evil characters with unique melee takedowns, but I can't say I mind.
Resident Evil stopped being a survival-horror series quite a while ago. Look no further than Resident Evil 4's Mercenaries mode for proof that, despite its origins, the series makes for a pretty solid arcade-style action game. Well, one modder decided that if Resident Evil is going for fast-paced shoot-em-up gameplay, it might as well go all the way back to the beginning: DOOM. A modder by the name of DooMero is working on a project to combine the Resident Evil Mercenaries gameplay with the environments of the original DOOM, and it works surprisingly well.
The mod includes all five characters from Resident Evil 4's Mercenaries mode: Leon, Hunk, Krauser, Ada, and Wesker, each with their expected weapon loadouts and special takedowns. The mod uses Resident Evil's gameplay, pulling the camera back to a third-person perspective with over-the-shoulder aiming, and somehow manages to drop it into DOOM's levels without looking out of place. You'll rack up points by shotgunning demons, just like the old days, but the fresh mechanics give you a whole new perspective on the classic DOOM experience.
DooMero lists the mod as 85% complete, but the gameplay itself is mostly finished. If you're interested, you can grab the mod for yourself right here from Mediafire. It isn't standalone, though: you'll need a copy of DOOM as well as its multiplayer mods, either Skulltag or Zandronum, for the mod to run properly.