Here's two guys playing Mortal Kombat II. In a Doom multiplayer match. The mod that lets you do this is still under development, and it's nowhere near as sophisticated as the one that lets you play Mario in Counter-Strike, but hey, it's Doom. With Mortal Kombat. [Thanks, DSOGaming!]
Could be wrong but this has been out or at least in a "beta" form for while. THey have also put together a make your own Donkey Kong map maker and player in Doom which is interesting as well. Doomero is my zDoom hero.
DK Doom link:
I really suggest anyone who is looking for fps fun to just spend some time looking around the zDoom forums. The quality of some of the projects is astronomical and it's all free. ( if you own the Doom 1-2 WADS which are cheap on steam)
Edit: Apparently the youtube videos for the project are down? Sadly it looks like they may have gotten a CnD though there isn't anything mentioned about it.