Bet you wouldn't see another DLC for this game huh?
Well your wrong because there is DLC coming with 1 new campaign, titled Cold Stream,
as well as the 3 old campaigns from Left 4 Dead 1, Dead Air, Blood Harvest and Death Toll.
The DLC is currently un-named, but Matthew Lourdelet is the man behind the amazing L4D2 Evil Eyes campaign, a mod exclusively only for the PC community. His latest campaign- Cold Stream. It isn't finished yet so dont expect it to be released any time soon, but Valve reckons it is even more promising than the Evil Eyes campaign. The campaign will not fit into the current storylines for either games, and will be availiable for Xbox and PC platforms.
The very first step in creating a new DLC is deciding on the larger goals and constraints for the DLC. The Sacrifice and the Passing were story heavy, so we want to see what the other end of the spectrum looks like. For this DLC we start with wanting to experiment by releasing a map pack that wasn’t about story. It would just be new campaigns for Left 4 Dead 2.
More details on the DLC haven't been announced yet.