Gonna have to say Kinkaku + Ginkaku. Because I like them and feel they could carry a story. Nothing overly long.
They used to be heroes. Something changed with them at some point. Seeing the coup d'etat and how it went down with Tobirama and Raikage. Databook said they used their tools in combination with their Kyuubi chakra. So probably a good fight. Why they turned on their village all of a sudden, possible influence of Kurama's chakra, corrupting them. What the hell clan are the from. Seeing them transition from loved heroes to disgraced criminals.
Databook also stated they committed numerous crimes in addition to the coup d'etat, so what exactly did they do? Bandits? Bounty hunters? Which brings up how and where they met Kakuzu - if they fought against each other, or worked together. Kakuzu was also there when Ginkaku was killed. What's that story. Who killed them. More of their abilities outside of the weapons, because databook states as Rikudou descendants, they had natural abilities that were of high value. Looking at the characters they're based on, Ginkaku could have been made a proficient earth-style user, in addition to the lightning most Kumo nin seem to be able to do.
And confirmation on their fate. They could still be sealed away. But given what we saw, they got out of the pots after the war. Or Kinkaku did at least. Is it possible they got purified and may have ended as heroes again. The Edo jutsu story seemed to give closure to deceased characters. If the chakra corrupted them, they could have been cleaned out with the statue sucking it out of them.
But this is my bias. I liked KinGin. Tobirama and Kakuzu are cool as well, so they'd be involved in their story. A's father may also be -could have been who killed both of them.
These 2 are favorites of mine that we didn't see much of. Other characters I really like have done a good bit, and still got some filler stories added in. Kakashi, Jiraiya, Orochimaru. However, given how unpopular these 2 are, it'd be no shock many won't care for their story. But they'd have other known and liked characters in it.
