A plot twist could be that the transformation into a girl makes Spiderman lose some of his traditional powers but gain new ones. For instance, instead of his usual web - slinging ability, he could have the power to manipulate plant life to his advantage. This would completely change the way he approaches fighting crime and protecting the city.
His relationship with Mary Jane would become very different. Instead of a romantic interest in the traditional sense, they could become more like sisters or best friends. Mary Jane might help the girl - Spiderman adjust to her new identity.
Well, in a fanfiction, it could be due to some sort of magic spell gone wrong. Maybe a villain was trying to defeat Spiderman in a new and unique way and accidentally turned him into a baby. Another possibility is that some strange scientific experiment in the Marvel universe backfired on him.
His identity might be at risk. If people find out that Spiderman has turned into a baby, it could expose his secret identity. Also, his friends and family would have to take on new roles. For example, Aunt May would have to be more of a caretaker than ever, and his superhero friends would need to protect him from villains who might see this as an opportunity to get rid of Spiderman once and for all.
One possible scenario could be that in this fanfiction, the change forces Kratos to view the world from a different perspective. As a girl, he might face different challenges and reactions from the other characters. Maybe the gods who once feared him now underestimate him, leading to some interesting power dynamics.
Perhaps in a parallel universe, Superman is born as a girl. This version of her grows up with different experiences and challenges. She could be more in tune with female issues in society and use her powers to fight for gender equality as well as the usual superhero stuff like saving the world from villains and disasters.
Another possibility is that Finn - now a girl - discovers new powers or abilities that were hidden before. Since in many stories, a change in gender can sometimes bring out different aspects of a character. She could use these newfound powers to fight against the villains in Ooo in a more creative way than before.
Maybe in this fanfiction, Midoriya as a girl discovers new aspects of her power. Since her body has changed, the way she channels her quirk might also be different. She could then have to train all over again to master this new way of using her ability.
Physically, of course, there would be obvious changes. But personality - wise, he might become more in touch with his emotions. In the original, Lelouch was often very calculating and hid his true feelings. As a girl, society in the story might expect her to be more open about her emotions, which could force her to confront and express them more.
One idea could be that Steven turns into a girl due to a magical mishap. Maybe he was trying to help a gem with a spell and it backfired. As a girl, he could struggle with how others perceive him differently, but also discover new aspects of his personality. He might have to learn new ways to fight and use his powers, adding an interesting twist to the story.