Warm up that cup of speculation, as it appears Valve are working on another game that seems to be going by the name of Citadel (likely a code name).

Linking into Half-Life, since the Citadel is the HQ from where the Combine govern Earth. Apparently though, this is entirely separate to the unannounced Half-Life VR game with Citadel being a completely different Source 2 project. As always though, do not take this as any form of confirmation. You can see the video from Valve News Network below which is quite interesting:

I've no doubt that all the "leaks" that end up shipping with Valve games are completely intentional. They probably love seeing everyone try to guess what it's going to be.

Will it be Half-Life 3? Probably not. Considering everything talked about in the video, it sounds like some sort of top-down game, possibly including some stealth elements. However, Valve are supposed to be launching a flagship VR game this year, which is likely Half-Life VR.