One idea could be that it's a sign of their growing closeness. Maybe it starts as a joke, but then it makes Harry realize his deeper feelings for Hermione. It could also be a way for Hermione to show her support for Harry during a really tough Quidditch season, and the team takes it as a good luck charm.
Perhaps in the fanfiction, Hermione wears the jersey during a special event at Hogwarts. Like a big Quidditch celebration where there are different houses competing in non - traditional Quidditch games. Harry is initially embarrassed but then proud, and this leads to a whole new dynamic between them and their friends.
Perhaps it's a sign of admiration. Harry is a great Quidditch player, and Hermione wearing his jersey could be her way of showing that she admires his skills on the pitch. It's like wearing a shirt of your favorite athlete in the Muggle world. This admiration could also be part of a larger theme in the fanfiction about how they all look up to each other in different ways.
It could symbolize a deepening of their friendship. In fanfiction, when Hermione wears Harry's Quidditch jersey, it might show that she is proud to be associated with him. It's like she's showing support for him in a very personal way. Just as a sports jersey in the Muggle world can represent team spirit and loyalty, here it could mean Hermione is loyal to Harry on a whole new level.
A possible plot is that there's a mysterious illness spreading among the Quidditch players, and Harry and Ginny have to figure out what's causing it and how to stop it before the big Quidditch tournament. Or perhaps a new and dangerous Quidditch move is introduced by a visiting team, and Harry and Ginny must learn it quickly to stay competitive. They could also have a friendly rivalry with another couple who are also big Quidditch fans.
Perhaps in the fanfiction, there's a school event where they have to swap clothes as part of a charm challenge. Hermione wearing Draco's clothes makes her see the Slytherin world from a different perspective. She might notice the small details in his clothes that reflect his status and family, which could lead to a deeper exploration of the Slytherin - Gryffindor divide.
One scenario could be after a big Quidditch match where Draco's team wins. Hermione, in a moment of celebration and a newfound sense of unity among the houses, puts on his jersey. Another could be if Draco gets injured during a match and Hermione, out of concern and a strange new bond, wears it as a sign of support. Or maybe in a magical prank gone wrong that results in a spell binding them in some way, and as part of the chaos, Hermione ends up wearing Draco's jersey.
One idea could be a story about a new Quidditch team formed by students from different houses. They face challenges not only on the pitch but also in terms of getting along with their diverse teammates.
One idea could be that Harry discovers a magical dress in the Room of Requirement that gives him special powers. Another could be that he wears a dress as part of a disguise to infiltrate a wizarding event. Or perhaps he is cursed to only be able to wear dresses for a period of time and has to deal with the reactions of his friends and foes alike.
Another plot could revolve around a conspiracy to fix the Quidditch World Cup results. A group of dark wizards might try to influence the outcome for their own gain, like betting on a certain team to lose. A young wizard or witch who discovers this plot then has to find a way to expose it and save the integrity of the tournament.
A third plot could be that their presence in Ravenclaw changes the overall perception of the house. Usually seen as more cerebral and less action - oriented, Harry and Hermione bring a sense of adventure. They might convince their Ravenclaw housemates to join them in more dangerous quests, like exploring the Forbidden Forest for a rare ingredient for a potion that could help in the fight against the Dark Arts.
One plot idea could be that Harry, Hermione and Susan team up to solve a new mystery at Hogwarts that the teachers are overlooking. For example, a series of strange disappearances of magical creatures from the Forbidden Forest. They use their combined skills in magic, Hermione's research abilities, Harry's courage and Susan's unique knowledge of certain spells to track down what's happening.