Left 4 Dead 3 and Source Engine 2 have been simultaneously spotted and snapped on a computer screen during a tour of Valve's offices conducted for the DOTA 2 tournament.
The image appears to be a changelog pertaining to various Valve games, including most notably of all "L4D3". Is Left 4 Dead 3 currently in development? And is the absence of Half-Life 3 on the list an indication that we're still going to have to wait an eternity to see anything of Gordon Freeman's long-awaited return?
Whatever the case, this is quite telling. It looks as though a Source Engine 2 powered Left 4 Dead 3 could well be on the cards, and the smart money says it'll land on Xbox One if this proves to be legit. Watch this space.

Source: http://www.xbox360achievements.org/news/news-15741-Left-4-Dead-3-Spotted-During-Valve-Studio-Tour.html
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