My own personal choices for spinoff films would be, in descending order:
1. A Fugitive Jedi "chase" film where a survivor of the Great Purge flees Darth Vader and Imperial troopers.
2. Yoda as a relatively young and brand new Jedi Master, set 800 years before the Prequel Trilogy when he takes on his very first Padawan apprentice and has to embark on a mission of urgency to defend the Republic from some sort of danger.
3. One of Boba Fett's early exploits as a professional bounty hunter during the formative years of the Galactic Empire.
4. Han Solo, set some seven to ten years prior to his meeting Luke, Obi-Wan and Leia and joining the Rebel Alliance.