Maybe a character who was experimented on by Orochimaru. The experiments accidentally give them the Rinnegan, but Orochimaru doesn't fully understand its power and neglects it. The character then breaks free and starts to explore and master their Rinnegan powers on their own in the fanfiction.
A character from a minor clan that has some distant relation to the Sage of Six Paths could be the main character. They inherit the Rinnegan but because their clan is so small and insignificant, their power is neglected. In the fanfiction, they could prove that their small clan can be a major force with the help of the Rinnegan.
It could be an unknown Uchiha who was overlooked in the main story. Maybe they were born with the Rinnegan but due to some political unrest in their village, they were forced to hide their power and were thus neglected. In the fanfiction, they could come out of hiding and shock everyone with their Rinnegan - based abilities.