Now, you have to ask if HL3 could ever hope to meet the expectations of the modern gamer, many of whom were probably just born when HL2 was big and have no real connection to it. What do they have to gain from it? What do they have to lose? I'd argue that they're making so much money now they don't particularly need the expense and risk of developing a game that, even if it is quite good, stands to potentially harm their reputation.
Aside from Steam, Valve (the developer) seems to be putting a lot of focus on esports right now. Many of the recent updates to Dota 2 seem to be new features and quality of life improvements for casters and spectators.
There is no way in hell they could release it as a VR game and not as regular FPS game. That would be just really bad move from Valve.