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.
Dumbledore, even if not in person, could have left behind some clues or resources that help Harry. Also, the Order of the Phoenix members who are still around and believe in Harry's cause. They have their own skills and connections that could prove useful. For example, Moody could teach Harry some advanced defensive magic techniques to use against the Ministry's security measures.
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.
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.
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.
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.
Well, perhaps Harry has finally had enough of the Ministry's corruption and inefficiency. So he decides to take it down using his powerful magic skills. He might start by exposing all the dark secrets hidden within the Ministry walls, which could lead to chaos and the downfall of those in power.
Well, maybe Boruto's family or friends are hurt by a powerful enemy. So he decides to train hard and seek revenge. He might travel to different places to learn new jutsu and gather allies along the way.
It could be about Sheldon getting back at someone who wronged him in the series. Maybe it's Leonard who ate his last slice of pizza without permission, so Sheldon devises an elaborate plan to make Leonard's life a little chaotic for a while.
Well, it could be about Issei seeking revenge on those who wronged him in the past. Maybe someone bullied him or betrayed his trust, and now he's out for payback. He might use his unique skills or powers to get even.