My bet? They'll never make either a HL2 ep 3 or a HL3.
Not because it wouldn't be successful - it would. But because it would harm their wider business interests.
Valve makes a lot more money these days from running what is in essence a platform than it ever made from being a games developer. Steam is a big and successful platform. Numbers relating to its success are notoriously hard to come by, but by joining together a few pieces of publisher and charts data (which exclude Steam sales) and feeling out the gaps, you can work out that in the closing year of the PS3/Xbox 360 console cycle, Steam was managing major games sales on a par with either of those consoles, while probably managing a lot more sales of small indie titles.
A big part of running a successful platform is managing your relationship with the wider industry - publishers in particular. Historically, in console land, Sony has been particularly good at this and Nintendo has been particularly bad, with MS somewhere in the middle. Valve is, by all accounts, pretty good at it. Almost everybody publishes on their platform. EA is trying to make a go of their own alternative with Origin, but that's hardly turning into a stunning success. Ubisoft thought about making a break for it with uPlay, but have relented and uPlay has just ended up as a pointless and inconvenient "wrapper" for Ubisoft games which often requires Steam to be running in the background anyway.
And a big element of having a good relationship with publishers is being seen by them as a partner, not a competitor. Since Steam first started to get momentum, Valve has confined its first-party games development to titles outside of the major commercial arenas. Portal and its sequel exist more or less in isolation in genre terms (at least outside of the indie market). Left 4 Dead was like nothing else around when it launched (though others have copied it since).
But if Valve were to release a major high-profile mass-market shooter, like another Half-Life, then Activision and all of those other companies who publish on Steam at the moment might start to look at Valve differently. All of a sudden, they're getting nervous about being reliant on a platform owned by somebody who is competing with them. Worried that their visibility on the platform will be reduced, or that they might get shunted onto the ass-end download servers if they launch in the same window. Why do you think non-EA support on Origin is so poor, despite EA being happy to carry other publishers' games?
It's the same over in console-land. MS and Sony do develop and publish first party games, but they're pretty blatant about the fact that they basically do it just to build the installed base of the console (making it more attractive to third parties). Their main revenue is from third party licensing fees, so the last thing they want to do is get into a cut-throat competition with those third parties. Nintendo, on the other hand, make first-party publishing a huge part of their business, which makes their platforms a scary place for third parties.
So yeah. Steam is great and all that. But it's probably killed off any prospect of more Half-Life for the immediate future.
The only way I could ever imagine that lock being broken would be with Half-Life 3 as a Steambox launch exclusive to give the console's installed base a flying start (so essentially acting as a loss-leader for the sake of third parties).