It could be part of a magical spell. There might be a spell that requires Hermione to wear Fred's clothes in order to unlock a hidden power or solve a mystery. And as she wears them, she starts to feel a connection to Fred on a deeper level, which then drives the story forward with new emotions and interactions.
One reason could be that they are in a situation where they need to disguise Hermione as Fred for some sort of mission. So she has to wear his clothes to blend in. Another possibility is that it's a symbol of their closeness. After a particular adventure where Fred saved Hermione, she wears his clothes as a sign of gratitude and a growing bond between them.
Maybe it could be a story where Hermione is cold and Fred offers his clothes. Then it becomes a cute moment that leads to a deeper friendship or some unspoken feelings between them.
She might wear his clothes because of necessity. For example, if her own clothes got damaged or lost during an adventure. Harry's clothes would be readily available and a practical solution.
Fred could use his knowledge of the Weasley family's magic tricks and inventions. He might have some secret gadget or spell that can break through the barriers or spells that are being used to torture Hermione.
Yes, there might be fanfictions out there with this concept. Fanfiction writers often come up with creative and unexpected pairings. However, this is a rather unusual pairing considering Hermione is typically associated with Ron in the original 'Harry Potter' series. You can try searching on major fanfiction websites like Fanfiction.net or Archive of Our Own. Just type in relevant keywords to find stories based on this idea.
One possible fanfiction could be where Fred and Hermione discover their Animagus forms together. Maybe they transform in the Forbidden Forest and have to rely on each other to get back to Hogwarts safely. Their forms could be something that represents their personalities, like Fred could be a fox, known for its cleverness and playfulness, and Hermione could be an owl, symbolizing wisdom.
A great idea could be a time - travel fanfiction. Hermione accidentally travels back in time to Draco's childhood. She has to blend in, so she borrows some of his clothes. Draco, as a child, is curious about this strange girl in his clothes. This interaction could change the course of their future relationship in the original timeline.
Well, in some versions, Fred might use his quick-witted magic skills. He could cast a distraction spell to draw Bellatrix's attention away from Hermione. Maybe a spell that creates multiple illusions of himself, confusing Bellatrix long enough for him to grab Hermione and disapparate them to safety.
Hermione, on the other hand, could become more daring. She is already brave, but this situation could push her to take risks she normally wouldn't. She might be the one to suggest trying untested spells or taking paths that are less known in order to escape. As they run, she could learn to trust her instincts more and not just rely on book knowledge.
One reason could be that they played a particularly mischievous prank on her that went too far. For example, if they made her late for an important exam or meeting with a professor by using one of their wild magical inventions. Hermione, being a stickler for rules and academic achievement, might lose her temper and hex them in retaliation.
In a fanfiction with this theme, it could be set post - war. Draco and Hermione find themselves in a situation where they are on the run from some dark wizards who are after them for different reasons. During their escape, Hermione gets wet and cold. Draco, having a change of heart after the war, gives her his warm cloak. This act of kindness begins to break down the barriers between them, and they start to have deeper conversations, realizing they have more in common than they thought.