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.
In the fanfiction, the Howler could be a warning from an unknown dark force. Dumbledore has been making moves to counter the rising dark magic in the wizarding world. This dark force, not wanting to be thwarted, sends a Howler as a form of intimidation. The Howler could contain veiled threats and references to past events that only Dumbledore would understand.
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.
In such a fanfiction, Dumbledore might face consequences for his actions. For example, his manipulative ways could be exposed. Maybe his secrets regarding Harry Potter's upbringing and the true purpose behind his actions could come to light, leading to a loss of his respected status among the wizarding community.
Well, it could be that Rumple discovers some past wrong Dumbledore did to him or his kind. Maybe Dumbledore once meddled in Rumple's magical affairs and now Rumple wants to make him pay. He could start by using his unique magic to create chaos in Hogwarts, like making the staircases go haywire or the portraits misbehave.
Well, perhaps in the fanfiction, Dumbledore is arrested because of his ties to a certain group that the Ministry has deemed as a threat. For example, he might have been secretly helping a group of werewolves or half - bloods who are fighting for equal rights, and the prejudiced Ministry decides to arrest him to stop what they see as a subversive movement. This could lead to a great uprising within the wizarding community, with students and other professors trying to prove his innocence.
In a 'Harry Potter gets revenge on Dumbledore' fanfiction, Harry could find out that Dumbledore knew about certain threats to his life but didn't do enough to protect him. So, Harry decides to get revenge by undermining Dumbledore's influence at Hogwarts. He might start by convincing the other students that Dumbledore isn't as infallible as they thought. He could do this by revealing some of Dumbledore's past mistakes. Then, he could work on getting the school governors to question Dumbledore's leadership, gradually chipping away at his power and status within the wizarding community.
In such a fanfic, perhaps Harry discovers some long - hidden, damning secret about Dumbledore. Maybe it's related to Dumbledore's past actions with Grindelwald. Harry, being the honest and justice - seeking person he is, feels compelled to expose this secret, which then leads to Dumbledore being fired from his position at Hogwarts.
In some fanfictions, Dumbledore is portrayed as having a lot of secrets. If he gets caught, it could be related to his past. Maybe some old enemies find out where he is and catch him. Then he would have to deal with his past coming back to haunt him. His friends might try to rescue him, but it would be a very difficult situation for him, and it could change the whole dynamic of the fanfiction story.
One possibility is that Dumbledore gets caught spying on a group of Death Eaters. This would put him in a very dangerous situation as the Death Eaters are extremely cruel and loyal to Voldemort. He might be tortured for information about the Order of the Phoenix. However, Dumbledore is very resourceful and would probably be able to hold his own against them for a while, giving his allies time to figure out a rescue plan.
It could be Severus Snape. He has a complex relationship with Dumbledore. Maybe Snape discovers some of Dumbledore's plans that he doesn't agree with regarding Harry Potter. He then sends a Howler in a fit of anger to express his disapproval.