Christopher Reeves
In my opinion, I think there should be a movie about Christopher Reeve, and his role as Super Man, and how this shaped who Christopher Reeve became. I believe that Brandon Routh is the only logical person to play Christopher in my opinion, and his parallel character should be highlighted as well.
Moreover, I think this documentary should also cover George Reeves and should use John Barrowman for this role. It may not seem a perfect match, but he is very similar in looks.
A History of Superman, and the actors who have had this role might be a great thought on this, and with all the Superman/Supergirl and other supers going on out there, I think it would be a well received movie!