We know about. The Rumor going around that Source 2 had been stealth launched with DOTA 2 Workshop tools.
Some of the Files in those Tools oddly look like game files to games like Left 4 Dead 3, Counter Strike 2, and Half-Life 3. There was also Left 4 Dead 2 and Team Fortress things in there.
Of course I'm just speculating what's going on here so take these with a grain of salt.
DOTA 2 was the test. The Test to see if Everything associated with Source 1 will be compatible with Source 2. Parts of the Rumor are that DOTA 2 is now running on Source 2. I don't remember that's actually confirmed or not, but if so that means the Test was a success and that's why we see Files of Half-Life 3, Team Fortress and Left 4 Dead 2 in the files. This leads to my hypothesis.
Orange Box 2 will be consisted of 3 games. Left 4 Dead 3, Counter Strike Global Offensive, and Half-Life 3. All using the Source 2 Engine and looking better than ever.
Left 4 Dead 3 will have Left 4 Dead 2 maps just like how Left 4 Dead 2 had both Left 4 Dead 1 & 2 maps.
CS:GO in Source 2 is just going with the flow of the latest trend of "Remastering" games. The Game doesn't need a remaster, but it may get the same treatment DOTA 2 did. As in the Game will be converted into Source 2.
Team Fortress 2, Will probably get the same treatment as well.
And Of Course the Big one is Half-Life 3, by now it's bound to happen, but when will it happen is the question.
All of this is speculation of course and the files in DOTA 2 may not have anything to do with Orange Box 2 or Left 4 Dead 3 or Half-Life 3 or whatever. But Source 2 seems to actually be a thing and Valve may show more stuff about at Gamescom so It's pretty exciting, I think.
Also I'm sure if Orange Box 2 is real It will be on PS4 and Xbox One thus justifying a CS:GO "Remaster" because ya'll just LOVE Remasters right?