Well...there's not much of an ongoing sort of story/saga to give it a subtitle, really.
They could name it after all the stuff everyone says is gonna make it great in the HL2 vs D3 thread:
"Half-Life 2: Wide Open Spaces"
"Half-Life 2: This Time, The Crowbar is Bumpmapped"
"Half-Life 2: Gordon Has No Clue What's Happening"
"Half-Life 2: Revenge of the Physics-Enabled AI"
"Half-Life 2: The Mouths Move Dynamically, But Idiots Didn't Stop to Think That it's Always Gonna Play the Same Conversations Anyhow"
There's other good ones too:
"Half-Life 2: Who Needs Nvidia?"
"Half-Life 2: Who the **** is this guy in the Suit?"
"Half-Life 2: We Swear, None of It is Scripted"
"Half-Life 2: Mod The **** Out of Us, Please"
"Half-Life 2: Gordon Loves Steam"
"Half-Life 2: The Fall of WON"
And of course, the ever popular:
"Half-Life 2: Why Do I Still Have This Lame Crowbar?"
and
"Half-Life 2: WTF, I Blew That Loser Guard Away For His Ammo, How Can He Still Be Alive In This One?!?"
In all seriousness, though, the game is probably just gonna be called Half-Life 2, at least as far as anyone's indicated.It'd be kind of weak with a subtitle, anyhow, if ya ask me.
Oh...Above was kidding, if no one noticed...no flames, please. :0\