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.
Interesting fanfics often play with the idea of Percy using his intelligence, which was shown in the books, in the context of the Scarlet Pimpernel's adventures. Maybe he uses his knowledge of wizarding bureaucracy to outwit his enemies. Also, the relationships he forms during his Scarlet Pimpernel - like escapades, whether it's with old friends from Hogwarts or new allies, add depth to the story. The settings, too. If the fanfic takes Percy to different magical or Muggle locations, it can create a whole new world of adventure.
In the Harry Potter books, Percy was often seen as a bit of a stickler for propriety and status. In the role of the Scarlet Pimpernel, this could translate into him being very careful about his public image while secretly being a hero. He might use his knowledge of wizarding society's expectations to his advantage when it comes to hiding his true identity as the rescuer. Also, his pride could fuel his determination to be the best at his new role, just as he was proud of his achievements at school and in the Ministry.
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.
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.
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.
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.
You could start by looking on popular fanfic websites like Archive of Our Own (AO3). Just search for 'Percy Weasley Scarlet Pimpernel' in the search bar and browse through the results. There might be some great stories there.
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.
Maybe in the fanfiction, there's a situation in a collapsing wizarding building. Fred is trapped under some rubble and Percy, using his skills and knowledge of magical architecture, manages to find a way to free Fred. As they escape, they start to realize how much they've missed having a proper relationship with each other, and Percy makes a vow to always be there for Fred.
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.