It seems that Valve can count to three after all, with one of the firm's lead writers Chet Faliszek revealing that a third Left 4 Dead game could be released "some time down the road".
When asked whether Valve would ever develop another Left 4 Dead installment, he responded with "A 3?", which was followed by "You know, some time down the road. Like anything, I would say that to all of our games. So that's not a promise of when or where."
Faliszek continued on to express his love for the Left 4 Dead franchise, which is one of his "favourite series. It's just so much fun with the characters and the world. I love zombies and I love horror. So, yeah."
In a commercial point of view pertaining to Left 4 Dead 3 coming into fruition, it would only make sense for a third installment to materialize with the first two iterations selling over 12 million units.