5. Count Dooku
Count Dooku is another character who already has a rich and extensive history when we first meet him in Episode II that remains relatively unexplored in the rest of the saga. It's often referred to by the other characters but never in great detail. As an ex Jedi master who wound up in league with Darth Sidious, an origin story might focus on a younger Count Dooku in his earlier Jedi career and depict an Anakin style downfall, although admittedly it would be a lot less dramatic and with less severe ramifications for the rest of the universe. The story would need to address how Dooku was seduced by the dark side in the first place, but it would be something I would be interested to see, particularly if it was very personal reasons like Anakin's that attracted him to the dark side as it would add another level to the character for me. While I think Christopher Lee is brilliant and his performance was excellent as the hero-turned-villain, there never appeared to be any emotional depth to his Sith lord character, an issue that an origin story could address very well.