I don't think anyone at Valve has even thought about Half Life in years. The problem is they want to do something revolutionary with each game, but at this point I just want them to finish the story. The Half Life series is by FAR my favorite video game series, and I just want them to wrap up the story and then end it. No cliffhanger, no "the battle is over, BUT THE WAR IS JUST BEGINNING NNNNGGHHHH-", just let it go.
Half Life was great because it was always very original, and continuing a series just to make money (or even just because the fans expect you to) is cynical. Just give us the last episode, bring everything involving the Combine to a close, and tell us who/what the goddamn G-man is! (and also let me remote control Dog and pummel some Combine please!)