By now the best way for valve to handle HL3 is by announcing and subsequently releasing a game called "Half Life 4", and act like 3 is old news, with all the "wait, you haven't played that?!" comments to boot. The game's story would be an aftermath scenario, in which characters keep referring to events that happened in 3, with enough detail to make you want to play through them, but vaguely enough that it keeps it's mystery. HL4 would completely close the story, and valve would "forget" about the franchise. When the credits roll, they would be overlayed on a picture of Gabe Newell's expressionless face, and there would be no music, just headcrab sound files playing, with an occasional wail of suffering.
it would be horrifying. and glorious.