One idea could be that Danny turns into a girl due to a ghostly spell gone wrong. In the fanfiction, he could struggle with the new body while still having to fight ghosts. Maybe his friends don't recognize him at first, leading to some comical and tense situations.
Perhaps there could be a love interest that is unique to his female form. Maybe someone who didn't notice Danny before as a boy, but now is attracted to him as a girl. This could add a whole new layer of drama and romance to the fanfiction, as Danny has to figure out how to handle this new relationship while still dealing with his ghost - hunting duties.
Many 'danny turns into a girl' fanfictions also include a journey of self - discovery. As Danny adjusts to his new form, he starts to learn more about himself and the concept of gender. He might start to question traditional gender norms and find new aspects of his personality that were suppressed before. This could involve things like trying out new hobbies or styles that he was previously hesitant to explore because of his male identity.
One popular 'danny turns into a girl' fanfiction might be 'Danny's New Identity'. In this story, Danny's transformation into a girl brings a whole new set of challenges and adventures. He has to deal with different social expectations and his own self - discovery in a female form.
In a Danny Phantom trans girl fanfiction, Danny could discover her true identity as a trans girl after an encounter with a special ghost. This ghost could be like a spirit guide that shows her visions of who she really is. Danny then has to figure out how to balance her newfound identity with her ghost - fighting duties. She may also have to deal with the reactions of her family, especially her parents who might be initially confused but eventually come around.
Perhaps in this fanfiction, Danny as a kid would have a different perspective on the ghost world. He might be more curious and less jaded. His appearance as a kid could also change how the ghosts interact with him. Some ghosts might underestimate him, which could be an advantage for Danny. He could use this to his benefit when dealing with them, while also trying to figure out how to turn back to his normal self.
Well, one possible plot could be that Danny, while on an underwater mission to stop some ghostly threat near the ocean floor, gets hit by a strange spell or artifact that turns him into a mermaid. He then has to deal with his new form while also trying to keep his ghost - fighting duties going. Maybe he discovers new powers related to his mermaid form that help him in his battles.
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.