Well, it could start with Marco waking up one day as a girl. At first, he's completely bewildered. He might try to hide it from his family and friends at first. But as time goes on, he starts to adapt. Maybe he even joins a new group of friends who are all girls and they introduce him to new hobbies and interests. The story could also deal with how he feels about his male friends now that he's a girl. He might have different dynamics with them, and some might even develop romantic feelings for him, which adds a whole new layer of complexity to the story.
The main antagonist could be a jealous classmate. This classmate has always been envious of Marco's popularity or abilities. When Marco turns into a girl, this antagonist sees it as an opportunity to make his life miserable. They might spread rumors or try to embarrass Marco in his new form.
One common theme could be self - discovery. When Marco is turned into a girl, he might have to re - evaluate his identity and how he views himself in the world. Another theme could be the reactions of those around him. His friends and family might have different reactions, which can create interesting storylines.
Well, I'm not sure of specific 'star turns marco into a girl fanfiction' stories off the top of my head. You could try searching on fanfiction - dedicated websites. They usually have a wide variety of such creative works.
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.