You could create a fanfiction where Tom Robinson's family gets justice in a different way. Maybe there's a new trial with more evidence that comes to light. It could also be interesting to see a fanfic where Atticus Finch has to defend someone even more controversial in the town, and how that affects his relationship with the community and his family.
A cool fanfiction idea might be to set the story in a modern - day setting while still keeping the core themes of the original. How would Scout's tomboyish nature be perceived today? And how would the issues of racism and prejudice play out in a more contemporary society? This could bring a fresh take on the beloved story.