I can't help but feel that Valve is up to something big.
I'm going to come right out and argue that Portal and Left 4 Dead are experiments and tech demos for something bigger, as much as they are self-contained gaming experiences. The technology that makes the Portal gun work and the "Director" AI program that governs the L4D pacing and spawning are both somewhat radical and yet also totally seamless; you're never "aware" of the complicated math that makes it possible, you're simply swept up in the experience.
More to that last point - the storytelling method and the notion of narrative in both Portal and L4D is incredibly unique for modern FPSs... it's never explicit, but rather subtle and environmental. Rather than throwing in a long opening cutscene full of exposition that means nothing to you and giving your player character a backstory, they simply drop you into a strange world and you learn about the world (and yourself) as you progress, and they manage to do this without succumbing to the worn-out "amnesia" cliche. The gameplay is incredibly tight and the pacing is perfect, so even if you're not paying attention to the story you're still having a good time; but if you take the time to explore, you are rewarded with all these clever little details that fill out the world without beating you over the head. Seeing "The cake is a lie" scrawled on the walls of a hidden room reveals far more about the Apeture Testing Facility than any voice recording or cutscene could ever accomplish.
I would expect to see some of this stuff used in HL2 Episode 3 - the last level of Portal certainly posits a link between the Portal universe and the Half-Life universe, so it seems pretty likely that Gordon Freeman will get his hands on a Portal gun at some point - but it wouldn't surprise me AT ALL to see Valve working on a totally new IP that uses all these technologies and methods (as well as other stuff we don't know about, and I'm sure they're going to beef up the rapidly-aging Source engine) to some other, grander purpose. Let's face it - Half-Life 1 had an unconventional narrative method but as the sequels have borne themselves out, the overall story arc isn't terribly absorbing, and they certainly couldn't start using these new techniques in a sequel without messing up Half-Life's DNA.
It also needs to be said that having Erik Wolpaw on Valve's payroll ensures that future Valve games will have, at the very least, a very twisted sense of humor.
I couldn't possibly begin to guess where Valve is going, but Valve keeps very close tabs on what people do in (and with) their games, and I suspect that they'll be very curious indeed to see how the public responds.