IF they were to make a movie about someone it should be either qui-gonn, count dooku, palpatine or obi wan training anakin between SW1 and SW2 (maybe even mace windu, but I am not keen on that)
Qui-Gonn: He is portrayed as a great jedi although we dont know much about him if you stop for a moment and think, it would be an interesting movie since he was dooku's apprentice and showed to be an expert force-weilder in his youth, thats why dooku took him.
Count Dooku: He was a great jedi who left the order since he didnt want to be part of the republic, he wasnt a sith yet, but then Qui-Gonn's death made palpatine convince him to join the dark side, apart from that and that he is and expert force weilder, we dont know anything else about him, he might have many stories we dont know of.
Palpatine: His youth is unknown by most of us, how became supreme chancellor, unknown too, he might have an interesting story to tell us that we yet don't know of?
Obi Wain training anakin between SW1 and SW2: It would be interesting what and how Anakin trained, also, since obi-wan started training him when he himself was a padawan still, it may show us to the pressure he was set to, a padawan training the person who is the chosen one, it may be interesting how he felt and how the rest felt towards him.