One idea could be that Percy is secretly working on a project to reform the Ministry of Magic from within. He might be gathering evidence of corruption in the ministry without anyone knowing. This could add a whole new layer of mystery to his character.
Another idea is that Percy is secretly a member of an ancient and secret wizarding society. He has to keep it a secret because the society's goals and activities are not fully understood by the wider wizarding community. This would give his character a lot more depth and intrigue.
Perhaps Percy has a secret friendship with a group of magical creatures that he's been hiding from everyone. He could be helping them with a problem that only he, with his knowledge of wizarding laws and regulations, can solve. It would show a different, more compassionate side of him.
Perhaps Percy's secret life is that he is a time - traveler. He uses his ability to go back in time to fix mistakes or prevent disasters, but he can't let anyone know as it could disrupt the timeline.
One idea could be Percy trying to understand the twins' pranks and finally joining in on one. Another could be the twins getting Percy to loosen up during a stressful time at Hogwarts, like exam week. Maybe they play a prank that backfires and Percy has to help them fix it without getting in trouble with the professors.
One idea could be Percy being sorted into Slytherin instead of Gryffindor and how that changes his relationships with his family. For example, he might have a more strained relationship with his Gryffindor siblings at first. Another plot could be Percy using his Slytherin cunning to outwit his political rivals in the Ministry of Magic later in life. Or perhaps a plot where Percy has to team up with some Slytherin students to solve a mystery that the Gryffindors are too head - strong to figure out.
Another plot could involve Percy being assigned to work with Hermione on a top - secret project at Hogwarts. Maybe it has something to do with protecting ancient magic texts. Along the way, they learn to respect each other's different approaches and form an unlikely friendship.
Maybe a fanfiction where Percy and his family get magically transported to a different era. They have to rely on each other and their family bonds to find a way back. Percy, being the rule - follower, might struggle at first with the lack of established rules in this new situation, but then he realizes the importance of his family's more flexible ways.
Another possibility is that his secret wife was an ordinary human at first. They fell in love in Percy's early days, before he became well - known. She's kept secret because Percy wants to keep her out of the dangerous world he often finds himself in. For example, she could work in a library, and they have a very normal - seeming relationship in private.
A third plot could revolve around Percy's family relationships. He could be feeling overshadowed by his more popular siblings. So, he decides to prove himself in a different way. He enters a magical competition that tests not only magical skills but also strategic thinking. Along the way, he learns to appreciate his family more and they start to see him in a new light.
A plot could revolve around Percy being sent on a mission by the Order of the Phoenix that is similar to what the Scarlet Pimpernel did. He has to assume a false identity in the Muggle world while also dealing with his own insecurities and the expectations of his family back in the wizarding world. Along the way, he meets a Muggle girl who he falls in love with, but he has to keep his true identity a secret from her, adding a romantic sub - plot to the fanfiction.
One interesting plot idea could be the initial shock and resistance from both Hermione and Percy. Hermione, being a strong - willed and independent witch, might fight against the marriage law at first. Percy, on the other hand, is very rule - abiding, so he might initially try to force the marriage to work just because it's the law. They could start off clashing over their different views on the situation. For example, Hermione might be more focused on finding a way to repeal the law while Percy is more concerned with following the regulations. As they spend more time together, they could gradually discover each other's hidden depths and start to develop real feelings for one another. Maybe Hermione realizes that Percy has a more creative side beneath his uptight exterior, and Percy discovers Hermione's vulnerability.
Perhaps there could be a time - travel plot where Hermione and Percy are sent back in time together. They have to rely on each other to get back to their own time. During this adventure, they see different sides of each other. Percy sees Hermione's quick thinking in a completely different context, and Hermione sees Percy's loyalty to his family and friends in a new light, which could lead to a romantic relationship.