An extremely unrealistic idea that I had in the back of my mind was not Left 4 Dead 3, but Orange Box 3. With the Orange Box 3, they'd be skipping number 2 and be releasing 3 core games inside the package
- Left 4 Dead 3
- Portal 3
- Half Life 3
All three roughly take place in the same type of world and has been confirmed by developers in the past. I wouldn't be surprised if this actually becomes a possibility.
In terms of the Left 4 Dead series, I think L4D3 is a definite possibility, mainly because they did the same thing with L4D1 and 2. The maps were ported to L4D2 from 1, along with player models and everything, which made everybody hyped.