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.
Well, Finn could start to notice how differently people treat her as a girl. Some might be more protective, while others might underestimate her strength. This contrast in treatment could fuel her determination to prove herself.
Well, one possible plot could be Finn, as a girl, having to deal with the different reactions from his friends. Some might be really supportive, while others could be a bit confused at first. Maybe he'd also discover new things about himself, like having a different perspective on relationships or challenges. For example, he could find that his usual approach to fighting monsters doesn't work the same way when he's a girl, and he has to adapt his combat style.
Finn might become more empathetic. As a girl, he could be more in tune with his emotions and those of others. For instance, he might notice small details about how his friends are feeling that he didn't before. He could also develop a different sense of style and self - presentation, which would be a new aspect of his character growth.
Finn's personality would probably become more brooding. The whole vampire thing is a big change, and he might start to withdraw a bit. He was always so full of energy and positive before, but now he could be more cautious and a bit more on the dark side. He might also become more territorial, like vampires tend to be.
Well, Finn might start having a craving for blood. He could struggle with his new vampire instincts while still trying to hold on to his old heroic self. Maybe he would try to find a way to reverse the transformation at first. But then he could also discover some new powers that come with being a vampire and use them for good, like having super speed to catch bad guys more easily.
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.
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.