Perhaps in the fanfiction, Izuku is dared by his friends to dress like a girl for a day. This could lead to all sorts of comical situations, like getting hit on by guys who don't realize he's a boy, or having trouble doing normal boy - related activities while in girl's clothing.
He might do it for a bet. Just like some friends might bet on who can pull off a crazy stunt, his friends could have bet him that he can't dress like a girl and act the part for a certain period.
One possible plotline could be Izuku adjusting to her new gender identity while still trying to pursue her dream of being a hero. She might face different challenges and prejudices in a male - dominated hero world as a girl. For example, some male characters might underestimate her at first but she proves them wrong with her skills and determination.
Maybe Harry could be on the run from some dark wizards and he disguises himself as a goth girl to avoid detection. He would have to learn how to act and move like a goth girl too, which could lead to some comical situations.
Sometimes, the source material itself can be a factor. If there are major changes in the official My Hero Academia storyline related to Izuku, it might make the author's original concept for the fanfiction no longer viable. Or perhaps they had planned the fanfiction to be part of a certain arc in the series, and when that arc didn't play out as they expected in the official story, they abandoned their work.
If Izuku is an alien, the antagonists could be other aliens who want to use his unique powers for their own evil purposes. Maybe they see Izuku as a key to conquering Earth or other planets.
One possible scenario could be that it's for a school play. Maybe Hogwarts is putting on a production where they need boys to cross - dress, and Harry is roped into wearing girl clothes for the role. It could add a comical element to the story as his friends react to his new look.
One popular scenario could be during a battle with a powerful villain. Maybe Izuku underestimates the villain's new power and gets seriously injured while trying to protect his friends. Another could be in a training accident, like when he's pushing his limits with a new quirk technique and something goes wrong.
One possible scenario is that she is a toddler and it's normal for toddlers to be in diapers. Her parents are simply enjoying a day at the amusement park with her, taking her on kiddie rides and making sure she is comfortable in her diaper.
Maybe the story could focus on how Izuku, as a girl secretly, has different perspectives and experiences compared to her male counterparts. She might face unique challenges in a male - dominated hero society, like having to prove herself even more. And there could be a sub - plot about her developing a romantic interest in another character, but being hesitant because of her secret.
It could be due to Bakugou's public shaming of Izuku. If he embarrasses Izuku in front of others too often, Izuku will reach his limit and snap.