I said L4D3. I know that this sounds a bit crazy, and a bit contrary to so many people yelling, "ZOMG, SELL OUTS!", but I think its logical. Unlike many, I accept your premise that Portal 2 and Episode 3 will be released either with each other, or close together. If we don't assume that, then the answers to this poll would be skewed so much as to be completely meaningless (Kind of like whats happening with people answering Half Life 3, likely reading it as Episode 3). So lets take the contenders...
Team Fortress 3: Team Fortress 2, presently, does not need a sequel. There is still a lot of life in TF2. They are still adding content, the style does not need next gen graphics to fit the cartoony nature, and a sequal would undo a lot of balancing work. I am kind of wondering how TF2, with all the support it gets, continues to be profitable: Then again, the job they have done with TF2 has garnered a lot of Valve love, and brand loyalty.
Counterstrike: I do love counterstrike, as a supplementary game. I played the first, and Source, for a long time, and greatly enjoyed it. It is still a great franchise. It's the essence of multiplayer shooters, distilled down to the core. And that's why I'm not so sure that it would stand up so well next to MW2 and BC2. Give those Juggernauts some time, and then people may want to play something as lean as Counterstrike. Since it is such a great, paired down shooter, it is the perfect multiplayer game to make when you want to showcase a new, fancy game engine. Make a Source 2.0, and I think it would be time for a new Counterstrike. Otherwise, it would just seem redundant to CS Source.
DoD: A lot of the same reasons as counterstrike, but with an addition: I don't think the WWII game glut has been out of style for long enough. Needs some more time before that can feel fresh again.
Half Life 3: I know you don't mean Ep 3, and are thinking post Ep 3. But i'm not prepared to wait for another episode. The episodic idea was interesting, but nowadays, the concept of DLC is a much better fit. The episodes need to just be over, and called a noble experiment. As for a Half Life 3...actually, this is second, behind L4D3. Half Life is the Valve Flagship. If a new engine is to be made, it needs to be applied to Gordon Freeman first. And Valve needs a new Engine. I'm all for the idea of design before graphics, but Source just looks dated these days. They're playable, they're even pretty at times, but it's showing its age. Valve needs a new engine. I just think that I need a steady influx of Valve games, and I would like a treat while I'm waiting.
Portal 3: Intriguing... but with Portals popularity, it screams with the risk of turning into a cash cow to be milked. A Sequel to Portal is a fantastic idea, particularly because the game was so sparse, there is still a ton of mechanics to explore. But after exploring those ideas in a full game, I think Portal should go on the back burner for a while. Let some time for ideas to germinate.
A New franchise is interesting...but I can't think of anything, really. One potential option though, and one that I would need to mull over, is a return to Shepard from Opposing Force. OF was a very excellent game expanding on the original Half Life, and doing the same with Half Life 2 would be awesome. Race x Aliens in Source? A little more information on what the G Man is up to? I would love to see Shepard again.
Ive never actually played Riccochet.
Now, L4D3. L4D is a game genre that is worthy of being explored in many regular incarnations. It's par for the course with Zombie movies to introduce new elements, and explore the emergent patterns that result. This sense of exploration fits perfectly. Try different weapons and locales. Try different special infected, and explore how they effect strategy, or team up with the old infected. It's hard to imagine a game that would be better for coming out with a new incarnation on a regular basis, with more mechanics to explore.