In my bailiwick, boxing, I see good potential in items that are not well-researched or understood at the present time. Things like appearance cards or bout-related cards. These were made for a single event, and they just did not survive in quantity. A few examples:
I also think there is room in top quality photos of major subjects.
Joe Louis, June 21, 1935, age 21. An iconic image of him. Same photo shoot produced this M120 Detroit Free Press premium.
Even Ali hasn't hit his full potential.