If Arthas turns into a girl, her relationships with male friends might become more complex. For example, some might see her in a new, romantic light. Her friendships with female characters could also change. They might be more empathetic towards her situation or be jealous of her new - found attention.
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.
Well, a possible plot could be Arthas, now a girl, embarking on a journey to find a way to reverse the transformation. Along the way, she teams up with other adventurers. She has to constantly battle her own insecurities as well as external threats. Maybe she discovers that this transformation is part of a larger, more mysterious magical plot that she has to unravel.
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.
Their relationship could be developed through shared memories. For example, they could both recall their time as students in Dalaran. Flashbacks of their innocent days together, studying magic and having friendly competitions, could bring them closer. This could lead to a rekindling of their old feelings in the present of the fanfiction.
One idea could be that Steven turns into a girl due to a magical mishap. Maybe he was trying to help a gem with a spell and it backfired. As a girl, he could struggle with how others perceive him differently, but also discover new aspects of his personality. He might have to learn new ways to fight and use his powers, adding an interesting twist to the story.
A possible fanfiction plot could involve Sherlock's friends' reactions to his transformation. Watson might be initially shocked but then adapt and see that Sherlock is still the same brilliant mind, just in a different form. Mycroft could be more concerned about how this transformation affects Sherlock's standing in the intelligence community. And Lestrade might be confused but still rely on Sherlock's skills to solve difficult cases, albeit with some awkward moments at first due to the change.