A possible plot could be that Mariep Dean's transformation is temporary. While she is a girl, she experiences things like first love or girl - only events. But she is desperate to turn back to her original self, so she goes on a quest to find a way to reverse the change.
It could be a witch who cast the spell that turned Dean into a girl. This witch might have some ulterior motives, like using Dean's new form to disrupt the balance in the hunting world. Maybe she wants to create chaos among hunters by making them question Dean's abilities now that he's a girl.
One way the story could develop is that Mariep Dean wakes up one day to find herself in a girl's body. She then has to navigate a world that now treats her differently. For example, she might face new social expectations at school or in her community. She could struggle with how to interact with her friends who now see her as a girl.
One idea could be that Dean, as a girl, has to navigate the same dangerous situations but with a different set of challenges due to his new gender. For example, he might face different reactions from male characters who underestimated him before. Another could be exploring his relationships, like how his friendship with Sam might change, with Sam having to protect 'her' in a different way. And perhaps there could be a plot where 'female Dean' has to use her newfound charm to get information in a case in a way that male Dean couldn't have.
One great fanfiction with this concept is 'Baby Dean's Adventures'. It has a really cute take on how the other characters deal with a baby Dean. They struggle at first but then it becomes a heartwarming story of protection and family.
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.
Dean would probably be very protective. He's always been the big brother looking out for Sam, and with Sam as a kid, that instinct would be even stronger. He'd be constantly worried about Sam getting hurt and would keep him close at all times.