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 be that Rachel is arrested during a paparazzi - related incident. She gets into an argument with a really pushy paparazzo, things escalate, and she is arrested for disorderly conduct. She then has to deal with the media frenzy around her arrest.
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 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.
One possible plot is that the baby's father is someone unexpected. And Rachel has to deal with the reactions of her friends and family. She might also face challenges in her career because of having a baby at a certain point in her life. For example, some directors might be hesitant to cast a new mother. But Rachel, being the determined person she is, tries to prove that she can handle both motherhood and her acting career.
One possible plot could be that evidence of her illegal use of power and abuse in Hogwarts is finally uncovered. Maybe a student bravely comes forward with proof of her cruel punishments and false accusations. This leads to the Ministry of Magic having no choice but to arrest her.
Maybe Ryoga got arrested for being in the wrong place at the wrong time. For example, he might have been near a crime scene just as the police arrived and due to some misunderstandings, like his strange appearance or his confused state which made him act suspiciously, he was taken into custody.
In a Tsuna gets arrested fanfiction, it could be that a new enemy discovers a loophole in the law and uses it to have Tsuna arrested. This enemy might be from a rival family or an unknown organization. Tsuna's arrest would then be a way to start a new arc in the story. His friends like Gokudera and Yamamoto would likely be very determined to get him out. They could start their own investigation, and in the process, uncover secrets about the enemy and the legal system in their world. It could also lead to Tsuna making new allies within the prison or among the legal authorities who come to believe in his innocence.
Maybe he was framed for a crime he didn't commit. Some jealous rival could have set him up to get him out of the way.
One possible plot could be that Dumbledore is framed for a crime he didn't commit. Maybe there's a dark wizard who has managed to plant false evidence against him. For example, evidence that he was involved in some sort of dark magic plot. Another could be that the Ministry of Magic, fearing his power and influence, decides to arrest him on false pretenses to gain more control over Hogwarts. This could lead to a lot of chaos in the wizarding world as Dumbledore has many allies and supporters who would not stand for his unjust arrest.
One possible plot could be that mankae is wrongly accused of a crime. Maybe he was at the wrong place at the wrong time. For example, he was near a store that got robbed and due to some misunderstandings, the police think he was involved. So he gets arrested and has to prove his innocence throughout the fanfiction.