According to the leaders of the Left 4 Dead 2 boycott—who recently enjoyed a trip to Valve—there's more downloadable content already in the works for the original Left 4 Dead, stuff beyond the already announced "Crash Course" episode.
According to on and off Left 4 Dead Walking_Target, Valve is planning something for release for the original, but wouldn't specify exactly what that was. It could be "a new Special Infected, a new Campaign or new Weapon(s)." Vague enough for you? "It's definitely one of the three, perhaps more," he writes, if that helps.