I recently saw footage of the Puppet Master mod for Doom on Skulltag which allows players to take control of the demons themselves to attack another team of marines. Wouldn't that be cool if there was a mod like that for Doomsday? That kind of versus mode with the performance of the Doomsday engine would be absolutely fantastic.
It doesn't have to be an exact replica of Puppet Master, which has custom levels. Rather, it could be like L4D style, where a team of like two marines go through each level of a DooM episode, and the demon team could take control of the monsters placed on the map, and would have to kill the marines before they reach the exit.
I'm not program-savvy whatsoever, otherwise I'd try to make this thing myself, but someone should do it