3. The Grand Inquisitor
Depending on how old the Grand Inquisitor from Star Wars Rebels is, there could be a lot of ground to cover with him. The Grand Inquisitor is a Pau’an, which is an important factor. Pau’ans were nicknamed the “Ancients” because their average lifespan was 700 years. If the Grand Inquisitor is an older Pau’an, his lifespan is almost as long as Yoda’s. Even if he were half that age, it would stick him firmly in The High Republic era, meaning he watched the complete breakdown of the Republic and Jedi Order from their golden age to their darkest days. Fans have only seen The High Republic on screen in the preschool series Young Jedi Adventures, and we're waiting for the upcoming live-action show The Acolyte. Having a long-living character like the Grand Inquisitor could allow Tales of the Jedi to explore this new time period.
Granted, early Rebels reference books state he's the human equivalent of being in his mid-40s. That is wildly up for interpretation. He might look like the human equivalent of middle age, but for his species, who knows if that means he's centuries old. Also, the reference books are constantly being tweaked with the release of new shows and movies. Since Star Wars is a mythology, small details should be up for changes and retcons if they better a story.
His downfall is a fascinating one, too. The Grand Inquisitor was a Jedi Temple Guard, a title that, in my opinion, is one of the cruelest parts of the Jedi Order. The Order touted that Temple Guards were one of its highest honors. A Temple Guard had to give up all individuality. They wore masks to hide their identities. Their personal lightsabers were taken from them to give them a neutral lightsaber pike. It was total dedication to the Order and a lifelong commitment, which could be a long time for someone like a Pau’an who could live for centuries. Being unable to express one's individuality for that long could do a number on a person.
We also know the Grand Inquisitor was at the trial of Barriss Offee, and her words helped push him to turn to the Dark Side. Completely disenfranchised, his fall aided Anakin Skywalker turned Darth Vader to storm the Jedi Temple during Order 66. The Grand Inquisitor killed his fellow Temple Guards to clear the way.
There is also potential to showcase the daily lives of the Inquisitorius. These stories have been heavily featured in books and comics but not as much on screen. That would be a fascinating time to explore.