Maybe some of the Hogwarts professors. McGonagall, for instance, has been against Umbridge's cruel rule. She could gather support from other professors and present a united front against Umbridge. They could collect evidence of her wrongdoings and hand her over to the proper authorities for arrest.
It could be Harry Potter himself. He has been a victim of her tyranny and might finally find a way to expose her once and for all.
The Order of the Phoenix could be responsible. They are always fighting against the dark forces and those who abuse power. Umbridge, with her unjust ways, would be a target for them. They might dig up dirt on her and ensure her arrest.
The Ministry of Magic could be responsible. They might be jealous of Dumbledore's power and popularity. They could see him as a threat to their own authority and come up with some false charges to arrest him. For example, they could claim he was hiding information about a dangerous dark wizard.
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.
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.
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.
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.
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.
It could be the wizarding authorities. If they discover that Vernon has been abusing Harry Potter all these years, they might decide to arrest him for child abuse. Even though they are in the wizarding world and Vernon is a Muggle, they could still have some jurisdiction over the well - being of a young wizard like Harry.
Hermione is a must. Her intelligence and extensive knowledge of magic and Hogwarts rules would be invaluable. Ron would be involved too. He's Harry's close friend and has a bit of a mischievous streak himself. Fred and George Weasley are very likely to be part of it. They are known for their love of pranks and their dislike of authority figures like Umbridge. Luna Lovegood could also be included. She has a unique perspective and might come up with some really out - of - the - box ideas for Harry's revenge on Umbridge. Plus, her presence would add an interesting and somewhat dreamy element to the whole revenge plot.
In some versions, Snape uses his knowledge of the Dark Arts and Hogwarts' secrets. He might set up a situation where Umbridge is lured into a restricted area of the castle and gets into trouble with some magical creature or ancient magic, causing her to be removed from her position.