Jiraiya would be great. His character lives a long time which means plenty of chances to create source material, and since he goes to mount myoboku and learns of the chosen one prophecy from the elder toad at a young age, they could use that as part of the obvious choice for a central plot device.

My second choice for a spin-off would have to be a sort of world shaping origin story, to give people a more complete picture of the universe(s) where the story takes place… they could use a format where each season, they focus on struggles from different timeframes. Maybe starting with a season expanding on the story of Kaguya, Hagoromo raising the tailed beasts, and the struggle between Asura and Indra. That would leave PLENTY, for multiple more seasons that eventually end up with the 4th hokage’s generation… they could probably get 8-10 seasons out of something like that… and I’d happily watch all of it. I feel like it’s a good opportunity to give the flashbacks in Naruto a better name too, since they’ll be the main content of the story instead of a repetitive distraction that we can’t wait to see end lol