Half Life 3 (or at least episode 3) has to be next, it only makes sense. Valve has made all of their twos now. They made Half Life 2, then came out with Team Fortress 2, Half Life: Episodes 1 and 2, and now Portal 2, not to mention Left 4 Dead 2. Now that they've made all of their 2's it's time for a 3.
Theyre currently working on DOTA 2 but yeah maybe after that hl3, but they still need CS 2. Source was is and will always be garbeej
I've read little tid bits on Steam saying Valve is working on HL2: Episode 3. Also them confirming it will be the last of the HL2 series. Also read an article via Steam that they are possibly considering a meet up between Chell and Gordon or at least intertwining the two worlds. Valve said, (and I'm paraphrasing here) "It was no coincidence that they both existed in the same realm." I'll come back and edit this with the articles. EDIT: I couldn't find the one talking about Valve saying they are working on Episode 3, but this is the one that mentions a possible crossover: http://kotaku.com/5799661/valve-turns-to-singleplayer+plus-not-no-more-singleplayer-says-chief