Perhaps in the story, there was a misunderstanding about Sunshine's disappearance or harm. The authorities thought Rachel had something to do with it. Maybe Sunshine was missing before an important event and Rachel was the last person seen with her. The police misinterpreted the situation and arrested Rachel, even though she was innocent and was actually just helping Sunshine with some personal issues.
Well, obviously Rachel Berry and Sunshine. But also, other members of the glee club could be involved. For instance, Finn might try to defend Rachel, thinking she's innocent. Or Santana could be there, making sarcastic comments about the whole situation. Kurt might also be involved, perhaps using his connections to try and get Rachel out of jail. And of course, the adults in the story like Mr. Schuester could play a role. He might be shocked at first but then start to investigate to prove Rachel's innocence.
It could be an overzealous security guard. If Rachel was at a high - profile event and the security guard misinterpreted her actions as something threatening or against the rules, he could be the one to call the cops and get her arrested.
Perhaps in the fanfiction, Rachel is wrongly accused of stealing a valuable prop from the theater. She was just borrowing it for inspiration but there's a misunderstanding and she ends up getting arrested. Then she has to fight to prove her innocence.
It could lead to a major shift in the story and dynamics among the characters. Maybe the other characters would struggle to cope with the loss and it could change the entire tone of the fanfiction.
Finn could be involved if the fanfiction wants to explore a relationship with him as the possible father. He was an important part of Rachel's life in the original series, and his reaction to the news would be significant.
There are many possible fanfictions on this topic. One could be where Rachel is bullied for her over - the - top ambitions in the Glee club. Maybe some of the other members are jealous of the attention she gets from the teacher and start to spread rumors about her, which makes her feel isolated.
There are probably 'Rachel Berry fanfiction' stories that focus on her relationships. For example, a story could explore her relationship with Finn in an alternate universe where things turned out differently. It could show how they might have overcome more challenges together and grown even closer.
I'm not sure about the absolute best, but some popular authors in the Glee fanfiction community might be good places to start. You could look for authors who have a high number of views and positive reviews on fanfiction platforms. For example, some authors who have been writing Glee fanfiction for a long time and have a deep understanding of Finn and Rachel's characters are likely to create great stories.