Perhaps some of the students or teachers at Hogwarts. For example, if they find out that Dumbledore's actions have put the school in danger, like Hermione might figure out some of his misdeeds through research. They could band together to make him face the consequences.
It could be Harry Potter himself. After realizing how he had been used, Harry might decide to hold Dumbledore accountable. Harry has the power and the motivation, having been through so much due to Dumbledore's actions or inactions.
In some interpretations, one possible crime could be his manipulation of Harry Potter. He knew Harry was a tool to defeat Voldemort from the start and perhaps didn't fully disclose this to Harry, leading Harry into a dangerous life without his full knowledge.
Aspiring young writers who are fans of the magical world. They see Dumbledore as a rich character to base their fanfiction on. They could be students who are inspired by the detailed world of Hogwarts and want to contribute their own ideas about Dumbledore through writing.
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.
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.
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.
It could be Harry Potter himself. After all, he has been the subject of Dumbledore's schemes for a long time. Harry might finally see through Dumbledore's actions and decide to take a stand against him.
In some fanfictions, he might be exiled from the wizarding community. This would isolate him from the people and places he cared about, such as Hogwarts and his friends. It would also serve as a public condemnation of his actions.
The fanfiction could focus on Dumbledore's backstory. Fans might write about his early years at Hogwarts as a student, his friendships, and how he developed into the great wizard he became. It could also include his interactions with other magical creatures in more depth than the original series, such as having secret meetings with centaurs or merpeople to gain wisdom.
It's also possible that Dumbledore has been misinformed or is under the influence of a dark spell himself. In the wizarding world, there are many forces at play. He could have received false information that made him think this was the best course of action for Harry. Another aspect could be that he is trying to teach Harry a lesson in a very extreme way. Maybe Harry has been too reckless, and Dumbledore wants to make him understand the seriousness of the situation.
Dumbledore might give Harry advanced spellcasting training. He could teach Harry some of the more complex and ancient spells that are not commonly taught at Hogwarts.