Just remembered that Double Fine had that experiment that probably resembles how Valve works. Schafer had them all meet up and pitch game concepts, they took the ones they liked and broke into small teams to prototype them. From there they pared down the games further until they were left with the three or so they wanted to make and then fully developed those.
Take that short term experiment and just make it the basis of your company since, unlike Double Fine, Valve has no money worries. They don't need to make games.
... But they are making HL3 and statements concerning the death of traditional SP games don't mean the death of HL3.