A good documentary is also not just a compilation of rare videos. It should provide new little behind the scene stories no one knows, or at least a new point of view on well known stories. You can also make a good documentary without anything really new if you have some interesting, personal thoughts about the movies, their creation, the people who worked on it. Watching a documentary is a bit like reading a book: the audience needs the feel to have learned something or to understand a point of view in the end.