I’m certain we can immediately rule out Left 4 Dead 3 because turtle rock is already making a spiritual successor to left 4 dead (back4blood). I’m also ruling out another counter strike since CS GO is still a cash cow for them.
My guess is another VR game of sorts (maybe a new IP) or a revival of one of Valve’s old franchises: Day of Defeat, Ricochet 2? Or maybe they’re working on Team Fortress 3
I can see them doing a portal vr game to test the physical capabilities of the general population’s bodies to withstand consistently intense and disorienting movement.
Either way Valve making sp games = win. Plz be half life 3 
I wouldn’t mind seeing a new Counter-Strike game. Though I am not sure they would do this given how popular the one still running is. It would just be nice to see one with better graphics and with more features like controller-supported lobbies.
I was hoping for a new Left 4 Dead game when I heard about this but it doesn’t seem to be the direction they are going at all. Not sure what it will be.
I wouldn’t mind seeing another Half Life title come out. With everything they said in the interview and with what is already being worked on, I can see this being their next game.
Valve can’t count to three so it’s probably something new, rather than a sequel.