For any series that has been on such a long hiatus like Half-Life, the last thing they should do is drastically change. Change for the sake of it is not always good thing, just look at something like Banjo-Kazooie Nuts & Bolts as a prime example (good game, but a horrible sequel that screwed Rare up in the long run).The only rumor we have of HL2: Episode Three is that Valve is developing Half-Life: VR.
The Valve that made Half-Life is long gone, and if they make a VR game, it's going to unequivocally be shit (unless you're one of those fairies that freaks out in open-mouthed joy at characters returning in completely shit form like the Star Wars fanboys).
I know Valve's usual schtick (or at least, it used to be when they actually made games) is to reinvent the wheel, but throwing something as drastic as mandatory VR into Half-Life 3 would be such a kick to the balls for the poor bastards that are still holding out hope for it.
Then again, these are the guys who thought that Artifact was a good idea, so...