6. Darth Sidious/Palpatine
This would be a no brainer for an origin story I think. As the ultimate evil bad guy that reigns as the constant villain throughout the entire saga, I'd love to see a story that shows how Palpatine got involved with the dark side of the force. Naturally it's hinted at in Episode III where he recounts the tragedy of Darth Plagueis the Wise, thinly veiling it as an old Sith legend in order to manipulate Anakin, but I think it could make for an interesting story showing Palpatine's rise to power, both in terms of the force and politically, but not too much politics as we don't want another Phantom Menace fiasco. This could only be half of the story though, with the rest of the story showing Sidious' training of his future apprentices like Count Dooku and General Grievous, particularly if these spinoff origin films are to tie into the existing saga. As with the other characters in this list Palpatine's origins may have already been covered in the EU, but I still think it could make an interesting origin film showing the beginning of the villain that dominates the rest of the saga.