We do not know what it is with Valve but they seem to try their hardest to make their fans suffer whenever they game gets to the second installment. We have Portal 2, Half-Life 2, Left 4 Dead 2 and more. Valve will come out with a sequel to their popular game and then work to make their fans suffer for the rest of their life.
The fans would not be so unhappy if Valve just comes clean and announce that there will be no sequel or something along that lines but they won’t ever do that. There has been talkeding that Left 4 Dead 3 could be in the making but so far, we have not come across any new evidence that shows that the game is being developed.
However, even if it is not, there is still hope that Left 4 Dead 3 might still happen. According to Gabe Newell, for a game to happen in Valve, all they need is a group of people that are willing to work on the game. So maybe if we ask for it long enough, somebody in Valve would hear our pleas and decide to work on it?