A plot device that is often used is the discovery of new powers or abilities. When Nightwing turns into a girl, she might find that she has some unexpected powers or enhanced senses that come with this new form. This could be related to a more feminine energy or something unique to the female body in the context of this fictional world. Also, the plot could involve Nightwing's struggle with her own identity. She has to figure out if she's still the same hero or if this new form means she needs to redefine herself, both as a person and as a superhero.
It's hard to say who the 'best' is as it depends on personal taste. However, some popular writers in the fanfiction community might be those who have a deep understanding of both characters' personalities and can write engaging action scenes. For example, some writers who have been around for a while and have a large following might be considered good at writing their stories.
One common theme is friendship. They often start as colleagues or allies and their friendship deepens as they face various challenges together. For example, in many fanfics, they rely on each other's skills during 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.
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.
Archive of Our Own (AO3) is another great place. It has a very user - friendly interface. You can search for 'nightwing and power girl fanfiction' either by the characters' names or by using relevant tags. There are different categories and ratings, so you can find the type of story you're interested in.
One interesting storyline could be them teaming up against a common enemy from another dimension. They both have unique powers and skills, so seeing how they combine their abilities to fight would be great.