Personally I think the only possibility is Half-Life 3 at this point. If not, Episode 3 but MUCH longer than Episode 2. They won't be skipping anything, they just would be making Episode 3 a full game. Which is likely, as there are A LOT of things left open. A lot. Also, I know Valve is slow, but not this slow. They won't make 7 hours worth of game after this many years. I guess TF2 took 12 years... Dammit, now I may be wrong, and by extension, quite sad.
But! Portal 2 and L4D2 should show that Valve can make games at better rates with the same quality. I think that given their current work ethic, Episode 3 would be out if it were "just" an episode.