Not long ago some eagle eyed modders spotted that you could turn Left 4 Dead's AI director on in Alien Swarm with a console command. It turns out they were onto something. Valve have announced that there will be a new game mode, Onslaught, and that the all-knowing, all-seeing AI Director will be controlling the alien hordes.
In Left 4 Dead the AI director would constantly monitor the player's stress levels, based on a number of factors, including how beaten up they were, number of deaths, ammo remaining and the like. From the sounds of Valve's announcement , Onslaught mode will be no different. It will be interesting to see how the director balances the numerous alien types. The idea of a sudden and unpredictable storm of parasites fills me fills me with dread.
Speaking of dread, you like being repeatedly torn to pieces and eaten, right? Good, because Valve are also adding another difficulty level above Insane called 'Brutal'. There will also be a checkbox that will set friendly fire to maximum effect, meaning that flamethrowers will immolate your companions even faster, and misplaced Vindicator blasts will do their maximum, devastating damage.
Personally, Insane mode fit my definition of 'brutal' quite nicely, so the new difficulty mode ought to be unfathomably difficult. How difficult? Well, Valve had this to say: "Nobody on the Alien Swarm team has managed to complete a mission on it."
Good luck, everyone.