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.
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.
Maybe McGee lost his own clothes somehow. So he has no choice but to borrow Tony's clothes. It could be an accident like his luggage got lost or his clothes got ruined in an incident at work.
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.
Maybe it's a dare. His friends could have dared him to wear Tony's clothes for a day and he reluctantly agrees. And as he wears them, he realizes there's more to Tony's style than he thought.
To show a deeper connection between them. It's a simple yet effective way to visually represent their relationship. When Sakura wears Sasuke's clothes, it can signify that she is part of his world in a more intimate way.
It could be for a funny or imaginative twist. Maybe to make the chicken stand out or for a special event in the story.
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.
It's appealing because it shows a different side of the characters. We usually see Hermione as strong and in control, so seeing her vulnerable is interesting. And Fred, who is often portrayed as the fun - loving prankster, gets to show his caring side. It also allows for new interactions between two characters who don't have a lot of one - on - one time in the original series.
Their relationship would likely become more serious and responsible. They'd have to put aside some of their youthful antics as they now have a child to think about, even if it's from the future. There would be a new level of trust and understanding as they work together to deal with the situation.
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.