If a photo of an internal Valve changelog list is anything to go by, we'll have a third installment of Left 4 Dead before we hear anything about that other Valve game we desperately want news regarding. The image bove was taken by a group of Dota 2 players lucky enough to tour Valve's offices recently as part of the International 3 Dota 2 tournament. If you look closely, you'll notice a reference to "L4D3" as well as "Source2", which appears to indicate that the game will use Valve's long in development new engine.
Source Engine 2
For an engine that feels like it's been around for half of my life, Valve's Half-Life-2-powering Source Engine could be doing a whole hell of a lot worse. Even so, the likes of Portal 2 and Counter-Strike: Global Offensive probably aren't pushing your PC to take up a part-time job as a very large, angular stove top.
Point being, Source is past its prime, and gamers want to see what magical crowbar-rendering tech Valve can pull out of its hat next. According to Chet Faliszek, however, Valve's in no rush to make another big leap any time soon.