A photo that has been allegedly taken at the Valve's office by DOTA 2 players shows that Valve is working on Source Engine 2 as well as the third sequel of the Left 4 Dead title.
The photo was taken at the Valve's office by competitors in the Valve Dota 2 championships and is said to be a list of active projects that Valve is working on right now. In addition to some already announced titles and projects, the Source Engine 2 and the Left 4 Dead 3 are both on the list.
As you remember, Valve's Gabe Newell already noted that Source 2 Engine will come eventually and neither it or the Left 4 Dead 3 have not been officially announced yet.
The sequel for the Left 4 Dead 2 has been expected but Valve has been quite silent regarding it. Unfortunately, the snapped picture does not include Half-Life 3 which would certainly be a nice thing to see.
All we now need is Valve to officially confirm these two projects.
Source:
VG247.com.