My guess:
-Half Life 3.
It's been way too long for Valve to be working on a 5 hour game, Half Life 3 makes a hell of a lot more sense. Plus Half Life 2 and it's episodes were startinh to feel a little old, not bad by any means just not at innovite as they were, especially since they've spent the last decade being ripped off. It need's a new weapon set, a new enviroment, a few new enemies, a coat of paint to the graphics and something as game changing as the gravity gun. Anf for that you need a full blown sequel.
-Left 4 Dead 1 and 2 for the PS3.
Now that Valve has a dedicated PS3 team it makes a lot of sense to do, especially with Steamworks on PS3. Who knows, they may even sell the two in one package.
-A completely new IP.
This is what I want to see the most. I'm tired of sequels. This may even be Valves first step out of the FPS realm, they've already scated around interesting game types with Portal 1 and 2.