I like this idea, possibly under the a more concise title of "Composite Source". This could also add to the idea that has been suggested before of linked layers/folders/group for sources.
Example: I could create a group that is my web cam (with a chroma-key to remove the green-screen), with a image behind it that changes every so often. This could then be saved as a "Composite Source", and could be selected from a submenu like Global Sources are now. This would allow people to make complex, yet reusable components for their Scenes.