It could be part of a 'truth or dare' type of situation within their group of friends. Izuku is dared to wear Bakugou's clothes for a whole day. As he does so, he starts to understand Bakugou's fashion choices better and also how others perceive Bakugou based on his appearance. It might also make Bakugou a bit more self - conscious about how his clothes represent him.
Eventually, he might come to accept it in a grudging way. If Izuku is wearing the clothes for a good reason, like in a team - up situation where it's part of a strategy, Bakugou might tolerate it. But he'll still probably make some snarky comments about how Izuku better not ruin his clothes.
I'm not sure if there is such a specific fanfic available. Fanfic topics can be quite diverse, but this one seems rather unethical in the context of the characters' relationships. You could try searching on major fanfic platforms like Archive of Our Own or FanFiction.net to see if it exists.
The plot could involve Bakugou getting a quirk - related eye injury during a battle. As a result, he has to wear glasses. His initial reaction is one of frustration because he thinks it makes him look weak. But as the story progresses, he realizes that the glasses are a symbol of his resilience. He trains harder and becomes an even more formidable hero, all while sporting his new glasses.
There's a fanfic titled 'Bakugou's Hidden Care'. Here, the story is set during their training days. Bakugou notices Izuku struggling more than usual. He doesn't show it in the typical way, but he starts to subtly protect Izuku from overexerting himself and getting injured. It's a great exploration of their complex relationship.
A really interesting fanfic is 'The Unexpected Bond'. This story starts with Bakugou and female Izuku being paired for a project at UA. At first, they clash because of their personalities. But as they work together, they discover hidden sides of each other. The author explores their emotional journey in a very detailed way, and it's a great read for fans of this pairing.
You can try Archive of Our Own (AO3). It has a large collection of fanfics, and you're likely to find many Bakugou x female Izuku stories there. Just use the search function and type in the relevant tags.
All Might would probably be one of the first to try to stop Bakugou. As Izuku's mentor and a symbol of justice in the story, he wouldn't tolerate such violence. Another could be their teacher, Aizawa. He's strict and protective of his students, so he'd surely step in.
Bakugou might be initially shocked. He's used to Izuku calling him 'kacchan' for so long that it's become a normal part of their relationship. So, when Izuku stops, it could throw him off balance.
It's a really strange concept. In the original My Hero Academia story, Bakugou and Kirishima have a great friendship, and Izuku is also their friend. Cheating is a negative and unethical behavior, and it's hard to imagine these characters in such a situation. It might be an attempt by some fanfic authors to create drama, but it doesn't really fit the essence of the characters.