We do not know what it is with Valve but they just seem to like the number 2. Both Portal, Half-Life, Left 4 Dead and even Dota comes with the number 2 in their name and it does not look like Valve is ready to break the chain just yet.
Gabe Newell hosted an AMA on Reddit this month allowing his fans to ask him questions that they would like him to answer. One fan asked if there is any future in the Left 4 Dead franchise.
While he did not outrightly say now, he did not say yes as well. All he said was that with games like Left 4 Dead, it is a great platform for creating showed narratives but he also explains that somebody in Valve will have to be interested in creating the game before it will happen.
If the fans show them how much they really want the game, somebody in Valve might notice and start working on it. Who knows?