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.
In some fanfictions, Harry might get Umbridge arrested by exposing her illegal and cruel actions towards students. For example, he could gather evidence of her misuse of power in punishing students with the blood - quill and present it to the proper authorities in the wizarding world.
It might be because of her anti - Muggle - born policies. Fanfiction often explores the idea that Harry discovers Umbridge's discriminatory plans against Muggle - borns. This is completely against the values of the wizarding world, and Harry, having Muggle - born friends, would take steps to expose her and get her arrested for such bigotry.
Snape could face some backlash from the Ministry. If they find out that he was involved in getting rid of Umbridge, they might try to find ways to punish him or at least keep a closer eye on him. However, his skills at subterfuge would likely help him avoid any serious consequences.
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.