Perhaps the plot could involve Petunia having some sort of magical awakening or discovery that leads her to perform this transformation on Harry. In this fanfiction, Harry might then use her new female identity to infiltrate parts of the wizarding world that are more difficult for a male to access, like certain all - female magical groups. And this could lead to all sorts of adventures and revelations about the wizarding world's gender dynamics.
Another possibility is that it's an accidental result of a powerful magic clash between Voldemort and Harry. When their magic collides in a certain way, it causes an unexpected transformation on Harry, turning him into a girl. This would add a whole new layer of complexity to their relationship and the overall story.
Petunia's character could also develop in this fanfiction. If she turns Harry into a girl, she might start to feel guilty or responsible. This could lead to a change in her relationship with Harry. Maybe she would start to see Harry as more of a person rather than just a burden. And she could also develop some magical abilities further as she has now performed such a complex spell on Harry.
A great plotline could be that Harry, now a girl, has to navigate the world of wizarding fashion and etiquette which she previously ignored. She might be forced to attend more social events at Hogwarts, like balls or tea parties, where she has to learn proper behavior. And she could also use her new identity to her advantage when it comes to spying on the Death Eaters. They might underestimate her because she's a girl, but she could outwit them.
There could also be new characters introduced in the fanfiction. Maybe a female wizard who befriends Harry in his new form and helps him understand the female side of the wizarding world better. Or perhaps a villain who specifically targets Harry because of his unique transformation, seeing it as a weakness to be exploited.
One interesting plot could be that as a girl, Harry has to navigate the different social dynamics in Hogwarts. For example, he might be treated differently by his male friends who now see him as a girl. They could be more protective or act a bit awkward around him.
One possible consequence could be how he is perceived by his friends. Some friends might be accepting right away, while others could be a bit shocked or confused at first. For example, Ron might have a really hard time adjusting to his best friend looking so different.
Well, in some fanfictions, it often starts with Draco using a spell or a potion on Harry. Then, Harry has to deal with the sudden change. He might struggle with his new identity at first. For example, he could be confused about how to act as a girl in the wizarding world. There could be a lot of humorous situations, like when he tries on girl's robes for the first time and doesn't know how to walk in them properly. Also, his friends might have different reactions. Hermione could be more empathetic and try to help him adjust, while Ron might be completely bewildered.
Well, 'Girl - Harry's Adventure' is a great 'Harry turns into a girl' fanfiction. It shows how Harry, as a girl, has to deal with the expectations of the wizarding society which are different for females. Then there's 'Harry's Girlish Transformation' where the author explores how Harry's personality might change or stay the same during this transformation. And don't forget 'A Female Harry Potter' which has an interesting plot about Harry's new life as a girl in Hogwarts.
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.