I think it would be a very useful tool to be able to have, in the same vein as border, chroma key, and color filter, an option to add transparency to each individual layer. I'm specifically thinking about bands performing, and being able to do a graphics overlay over the webcam, but making it so that if they wanted to have that be a video file over the webcam, they could still be seen through. This would also apply to someone looking to play a game, and have their webcam be unobtrusive without requiring them to get a green screen to make that happen.