It could be Voldemort. He might have used his influence over some Death Eaters within the Ministry of Magic to frame Dumbledore. By getting Dumbledore out of the way, Voldemort could have a free hand to carry out his evil plans in the wizarding world.
There could be a group of pure - blood supremacists behind it. They view Dumbledore's inclusive policies at Hogwarts as a threat to their pure - blood ideology. They might work in the shadows to create a situation where Dumbledore is arrested, hoping to bring back the 'old ways' of the wizarding world where pure - bloods had more privileges and power.
The Ministry of Magic officials themselves could be behind it. Some of them might be power - hungry and see Dumbledore as a threat to their authority. They could fabricate evidence against him to get him arrested and thus gain more control over Hogwarts and the wizarding community at large. For instance, a corrupt official who wants to impose more strict and discriminatory laws might target Dumbledore as a symbol of resistance.
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.
For Lily Potter, her husband James Potter might be a likely ally. He would support her in any conflict she has with Dumbledore. Also, her close friends from her school days could side with her. They know her well and would trust her judgment.
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.
There's also a chance that Dumbledore gets caught by a group of rogue magical creatures. For instance, if he was exploring an area that was supposed to be off - limits to wizards and the creatures there saw him as an intruder. They might not be as rational as wizards and could hold him captive without really understanding who he is.
A fanfiction could be about Snape's unspoken loyalty to Dumbledore. Despite their differences and the difficult tasks Dumbledore assigned him, Snape remained faithful. This could be explored through Snape's thoughts and internal monologues, showing his deep - seated respect for Dumbledore.
Another possible antagonist could be a group within the wizarding world who doesn't want Dumbledore's plans to be exposed. They might have been benefiting from Dumbledore's secrecy or they believe that his plans, although seemingly unethical to Harry, were necessary for the greater good. They would then try to stop Harry from revealing the truth.
His friends like Ron and Hermione could be his allies. They have always stood by Harry and if Harry believes that Dumbledore has wronged him, they might side with him. Ron, with his loyalty and Hermione with her intelligence could help Harry plan and execute his revenge. They could use their knowledge of magic and Hogwarts to find ways to undermine Dumbledore's influence.
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.
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.
One great young Albus Dumbledore fanfiction is 'The Young Dumbledore's Adventures'. It delves into his early years at Hogwarts, his friendships, and his first encounters with magic in a really exciting way.