I've played L4D2 quite much recently, so I had this idea born.
There is a mod for Doom3, where you cooperate in group of 4 players to play original campaign. It's realy cool, as Doom 1-2 had this option by default. Also, Deathmatch for pretty single-playered game like Doom3 is too traditional to get quite much players (unlike initially multiplayer Quake Live or other examples). So I think that it would be pretty good if single player experience would take its part in multiplayer.
How it could be done? Well, pretty easily - there is already mod for zombie waves, there is mod for Coop, and there are a lot of mappers, creating their beautiful stuff. So, if we combine it, we can get multiplayer surivival campaign for Doom3. There's even no need to get some extra rules (like my idea of "Real Doom3" posted previously) or making possibility of playing with special demons (L4D style) or zombie evolution (Natural Selection/Tremulous/AvP2 style) to get initial game feeling (of course, extra stuff is welcome).
And you will get into quite classical today cooperation gameplay (Dow2: Last Stand, L4D, Payday, Portal2 etc) that requiers not only shooting but also covering and doing something together, which I find very cool, and that fits Doom3 survival feeling very much, IMHO. Like the Q3Defrag, but with frags )))