One idea could be that Harry discovers a hidden part of the wizarding world where there are 'monster girls'. Maybe he stumbles upon a secret club at Hogwarts. Through his interactions with them, he learns about their unique cultures and magic. This could lead to him wanting to be more involved with their world, perhaps even using his skills to help them with problems they face.
Harry's character can be developed by showing his curiosity. When he first encounters the concept of'monster girls', he could be intrigued and start asking questions. This shows his inquisitive nature which is a big part of his character in the original series. For example, he might ask about their magic that is different from what he knows at Hogwarts.
Maybe in the fanfic, Harry discovers a magical spell that can temporarily transform him into a girl. This leads to a series of adventures as he experiences life from a female perspective. He could face challenges like dealing with different social expectations for girls in the wizarding world, such as how girls are treated in Hogwarts classes or social events. It could also involve him making new friends who help him understand and embrace this new side of himself.
Hermione would be a great supporting character. She could be the one who first notices Harry's interest in becoming a girl and offers her support. She might share her knowledge of magic that could help Harry in his transformation, like spells for changing appearance or potions that can affect one's gender characteristics in a more subtle way. Also, she could provide emotional support, being a good friend and listening to Harry's concerns about how others will react.
One common theme is self - discovery. Harry has to figure out who he is when he has these new feelings about his gender. Another theme is acceptance, both from himself and from the people around him, like his friends and teachers at Hogwarts.
Perhaps in this fanfiction, Inuyasha is cursed by a powerful demon to become a girl. He has to find a way to break the curse while dealing with the new - found social and physical differences. For example, he may have to learn how to dress and act in a way that is expected of a female in his society. This could lead to some comical situations as well as some deep self - discovery moments.
There's a fanfic called 'Wounded and Yearning'. Harry is seriously hurt in a skirmish. His longing for Ginny becomes his strength. Ginny, sensing his need, rushes to his side. The story beautifully portrays their relationship as she helps him through his physical and emotional pain.
Perhaps Harry has a vision or a prophetic dream where he sees himself as a healer, saving lives. This drives him to study hard in his Healing classes at Hogwarts. He also starts to research ancient healing techniques on his own. He faces some challenges, like some people doubting his ability because of his past as 'the boy who lived', but he overcomes them and works towards his goal.
One popular fanfiction with a 'Draco wants Harry' theme is 'All Life Is Yours to Miss' by Saras_Girl. It's really well - written and delves deep into their relationship. The story is set in an alternative universe where their dynamic is explored in a very interesting way.
Voldemort might be initially thrown off by Harry's change. He may see Harry as a less threatening figure if he becomes a girl, thinking that the 'chosen one' prophecy might not apply in the same way. But then he could also see Harry as an even more interesting target, as a female 'chosen one' would be something new and unexpected in his view of the wizarding world's power dynamics.