Maybe a mysterious new character in the fanfiction. This character could be from Adrien's family's past, someone with a vendetta against his family, and decides to start by getting Adrien arrested to disrupt the family. For example, if Adrien's family wronged this character's family long ago, they see Adrien as a target to take revenge on the whole family.
Nathalie might be a character in these fanfictions too. Given her role in Adrien's life in the original series, she could be involved in the events leading up to his akumatization. Maybe she accidentally triggers his negative emotions. And then there's Gabriel, Adrien's father. His actions or lack of actions towards Adrien could be a major factor in Adrien getting akumatized. So, he could be a key character in the fanfiction, either as someone who realizes his mistake and tries to help or as someone who makes the situation worse.
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.
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.
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.
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.
Well, in a 'Percy Jackson Gets Arrested' fanfiction, perhaps he could be framed by a monster in human form. The police, not knowing his true identity as a demigod, might take him into custody. This could lead to a whole lot of chaos as his friends from Camp Half - Blood would surely try to break him out. They would have to figure out how to deal with the mortal legal system while also fighting off any monsters that might be taking advantage of the situation.
Perhaps Vernon is caught up in a wizarding - related incident without realizing it. He might accidentally help some dark wizards thinking they are just normal business partners. When the wizarding world realizes his unwitting involvement, they arrest him to question him about the dark wizards' plans. It could also be that his mistreatment of Harry finally catches up with him in a legal way. For example, if there are laws in the wizarding world about the proper treatment of magical children in non - magical households.