I think Valve is waiting to see if VR catches on in popularity, which may never happen if prices don't drop significantly. They were really dedicated to the technology for a while. Environmental destruction was pretty much perfected in Battlefield 1, so I don't think it would be much of an innovation. Episode 3 almost became a reality, there's even a leak with most of the assets put together. But I believe the next game will actually be Half-Life 3 instead, as Valve themselves seemed to tire of the Episode format. As for AI, we have yet to see how Valve will react to it, not very enthusiastic as they already put limitations for AI games.