Perhaps Harry uncovers a conspiracy where all his friends, the teachers at Hogwarts, and even the Ministry of Magic were in on a plot to control him. He then decides to take revenge in a very calculated way. He starts by sabotaging the Ministry's operations secretly. He might use his invisibility cloak to move around undetected and cast spells that disrupt their meetings or plans. For his friends, he could play mind games, making them feel guilty about their actions without revealing his true intentions at first.
It could be the Ministry of Magic. They have often been shown to be corrupt and self - serving in the original series. If they try to manipulate Harry again, he might see them as the main antagonists and target them in his revenge.
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.
Hermione would be a likely supporter. She is always loyal to Harry and if he presented her with evidence of Dumbledore's wrongdoings, she would probably side with him. She has the intelligence to help Harry strategize and find ways to expose Dumbledore. For example, she could use her research skills to dig deeper into Dumbledore's past.
Maybe Harry uses magic to make their house a mess. He could turn Dudley's room into a pigsty or make Aunt Petunia's precious china float and then smash. It's his way of getting back at all the years of mistreatment.
Perhaps Harry finds out that the Order has made a deal with someone he doesn't trust. In his revenge, he could expose this deal to the wizarding community. He'd use his influence as the Boy Who Lived to make sure the public knows. He might gather evidence and present it in a very public way, like during a big wizarding event. This would not only embarrass the Order but also make them lose the trust of many wizards and witches who previously supported them.
One possible plot could be that Harry discovers some deep - seated corruption within the Ministry. He starts by gathering evidence secretly. Then, he uses his skills and the help of his friends to expose the wrongdoings in a very public way, perhaps through a magically - enhanced news broadcast that reaches every wizarding household, causing great embarrassment to the Ministry officials involved.
Maybe Ginny did something that Harry thought was a betrayal, like siding with someone against him in a wizarding conflict. So Harry decides to play some harmless pranks on her to get revenge. For example, he might use magic to make her quill write silly things during a class.
Obviously, Harry Potter himself is the central figure. His friends like Ron and Hermione would also be involved. They would support Harry in his revenge plans, with Ron providing his loyalty and Hermione using her intelligence to strategize. Ginny Weasley might also be part of it, as she is Harry's love interest and a strong - willed witch in her own right.
Some fanfictions might have Harry getting revenge on the Ministry of Magic for their incompetence and corruption. He could expose their wrongdoings to the wizarding public and bring about a new era of transparency and fairness in the magical government.
In some fanfictions, Harry might use his magic skills in a creative way to embarrass Umbridge. For example, he could make her signature pink outfits turn into ridiculous colors every time she tries to be authoritative. Another scenario could be that Harry reveals some of Umbridge's darkest secrets in front of the whole school, which would surely be a form of revenge as she values her reputation so much.