Valve has also announced via the Left 4 Dead 2 web site that soon it will start adding Mutation modes created by the community. The first will be one made by Raymond Nondorf, a community member who created a Mutation mod that can be downloaded via the game's official message boards. And you can create your own Mutation via Nondorf's own tutorial. If you create one and submit it to Valve they could use it for a future Mutation that will be updated for both the PC and Xbox 360 version.
Valve also hints about future new content for Left 4 Dead 2 stating, " ... we should have some news for you early next year (it really isn't that far away) on any new content we might have in the works."
Gallery: Left 4 Dead 2 Screenshots