The extent of Left 4 Dead 2's mods already defies description, where the truly wacky (opens in new tab) shambles alongside the horde of new weapons and campaigns (opens in new tab) . Now, Valve wants to further slash at the barrier between a modder's creativity and what's possible in-game with the Extended Mutation System (opens in new tab) , an expanded scripting toolset and the future name of my progressive rock band.
As Valve explains it, the system "allows script authors to go past modifying existing scripts and write custom script logic, spawn and control entities, and much more." Think of entirely new game modes, a restructured Director, or custom HUDs. One of Valve's examples was a mode called Holdout that has survivors buying and erecting barricades against waves of undead similar to Call of Duty's Zombies spin-off.
The Extended Mutation System is in beta testing (opens in new tab) for now, but Valve's developer wiki (opens in new tab) already provides a few initial walkthroughs for sample custom modes. I'm keen on GnomeHunter (opens in new tab) —a CTF-meets-hot-potato mode where players have to lug that infernal garden gnome to a locker—but I'd love seeing a variation of Team Fortress 2's Prop Hunt in Versus mode. Why? Because the Hunter can't smell me if I'm a safety cone. Make it happen, modders!