The rumored "prequel" for Left 4 Dead between Valve and Team Overkill is about as accurate as a Boomer is fast. The original collaboration between the two studios came together as simple as they both like four-on-four multiplayer.
PCGameN talked to Valve at E3 and got the direct info on why the game isn't Left 4 Dead:
"We're trying to be a little coy and a little fun because we want the communities to discover it, but those guys are still making Payday: The Heist, we're still making Left 4 Dead."
So what are they working on anyway? It "is kind of akin to how in TF2 you might see some other games' weapons get translated and put into that game." Which means throwing bottles of urine at zombies or cops? Eating a sandwich to restore health? Uh, using an electrified shield to stop guys from literally stabbing you in the back?
Valve's answers are even worse than no answer at all. Because it furthers the idea that Overkill and Valve might be creating a hybrid of their two platforms--three if you dare put Team Fortress 2 into the blender--and see what comes out. A bank robbery that goes wrong into a zombie apocalypse? Sneaking into special facilities a~la opening of the Resident Evil film to steal a virus?
All of these possibilites are killing us, except for the truth that there is no L4D prequel by them.
Anything specific you'd like to see in the Valve/Overkill mystery game?