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.
Perhaps it could be a group of bullies from Issei's school days. They used to pick on him and now he's coming back to make them pay.
There could be consequences within the story itself as well. It might change the entire future arc of both Issei and Koneko in ways that are difficult to reconcile with the original character traits. For example, their relationship dynamics would be completely altered, and they would have to deal with the responsibilities of pregnancy, which could be a very different direction from what the original work intended. Additionally, other characters in the fanfiction might also be affected by this new development and their reactions could be hard to predict and write in a believable way.
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.
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.
She might get revenge because she felt the Order made a decision that put her or her friends in danger. For example, if they sent Harry on a dangerous mission without proper support, Hermione could plan some elaborate scheme to teach them a lesson. Maybe she'd use her intelligence to expose some of their secrets or weaknesses.
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.
Maybe Ron accidentally ruins Hermione's important experiment in the wizarding lab. In the fanfiction, Hermione could get revenge by getting all their mutual friends to side with her in an argument. She could present all the facts about how Ron's carelessness has cost her time and effort, making Ron feel isolated and guilty.
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.