First of all, I'd like to say to the folks at Valve: Making movies is hard.
Secondly I think the best thing Valve could do is make the animation movie, but then use that as the blueprint for a live action movie. They would basically have the movie scripted, story boarded, and camera angles, special shots, etc. all worked out. Then the experts in Hollywood could transform that vision into a live action film under Valve supervision. Then, everyone rakes in lots of money.