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.
One interesting plotline could be Harry getting help from a secret magical society that has been hidden from the wizarding world. This society might have unique spells and knowledge that they share with Harry to defeat Voldemort.
In some fanfiction, Harry might get married in a secret ceremony to protect his family from a new threat. His friends would be the only ones in on the secret. The wedding could take place in a hidden location, like an ancient, enchanted forest. His wife could be a powerful witch who has her own secrets. Their marriage could be a key part of a bigger plan to defeat an emerging evil. There could be a lot of mystery and intrigue surrounding their relationship, with them having to keep their true feelings and intentions hidden from the outside world.
One possible plotline could be Harry finding new and unexpected allies outside of the wizarding world he once knew. Maybe he discovers a hidden community of magical beings who are shunned by the mainstream wizarding society and they take him in.
One interesting plotline could be that Harry wishes for the return of his parents. This would completely change the dynamic of the story. He would no longer be an orphan and would have a family to support him. It might also change his relationship with Dumbledore, as he wouldn't be as dependent on the headmaster. Another could be that he wishes for an end to Voldemort without having to fight. This would lead to a very different kind of resolution to the main conflict in the series.
One possible plotline could be Snape getting revenge on those who bullied him during his school days. He might use his potions skills to create some harmless but embarrassing concoctions for them.
One popular 'revenge Harry Potter fanfiction' might be those where Harry gets revenge on the Dursleys for their mistreatment. In some fanfics, he uses his magic in clever ways to make them realize how wrong they were. For example, he might expose their lies to the neighbors in a magical but non - violent way. Another type could be Harry getting revenge on the Death Eaters in a more brutal fashion than in the original books, perhaps using dark magic he learned from some hidden source. However, it's important to note that these often deviate from the original themes of the Potter series.
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.
One interesting plotline could be Harry exploring the full extent of the Elder Wand's power without being corrupted by it. He might use it to restore parts of the wizarding world damaged during the war.
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.