Left 4 Dead 2 beta program has added the Extended Mutation System, an expanded toolset for modders to author custom scripts controlling enemy behavior, spawn points and custom game types. A Valve-created survival variant, called Holdout, is included as a Mutation example.
Valve has published a wiki page detailing everything possible with the Extended Mutation System for budding directors, explaining the scripting language used in the game and some of the finer points of Mutation modding. Throughout this test phase, Valve will monitor what Mutations players come up with, fix bugs when needed and keep a close eye out for any features that may be missing.
If you're in the Left 4 Dead 2 beta program – if you want in, this is probably the best place to ask for an invite – you can get cracking on your own custom Mutations right now.