It could be a mischievous student who accidentally casts a wrong spell. Maybe a first - year who is still learning how to control their magic and ends up turning Harry into a cat.
Perhaps all the main characters are transformed into cats at some point. Harry, Hermione, and Ron have to navigate the wizarding world as cats. They might use their cat - like abilities, such as stealth and agility, to solve mysteries and defeat dark wizards. The Hogwarts castle could also have new areas that are only accessible to cats, where they find clues and magical artifacts.
One possible plot could be that Harry, through some sort of magical accident or spell gone wrong, is turned into Daphne. He then has to navigate the world as a female Slytherin. He might struggle with the different social dynamics in Slytherin house compared to Gryffindor. And he has to keep his true identity a secret while trying to find a way to reverse the transformation. All the while, he could be learning things about Daphne's character and past that he never knew before.
The Dursleys could start by being even more cruel and abusive. They could constantly belittle Harry, tell him he's worthless and that he doesn't deserve any kindness. This could chip away at his self - esteem and make him more vulnerable to dark thoughts.
It could be Voldemort. He might use hypnosis as a new way to get to Harry, trying to make him a weapon against the Order of the Phoenix. Another possibility is a new dark wizard who wants to make a name for himself by controlling Harry Potter.
There could be. It might involve Harry Potter being magically transported to a realm where the Cheshire Cat is. The story could play with the different magical rules of the two worlds. For example, the Cheshire Cat's disappearing and reappearing ability might interact in an interesting way with Harry's spells. It could also explore how Harry's character would react to the rather chaotic and illogical Wonderland environment.
Neville is a likely candidate. He has always been brave and has a strong connection to the Gryffindor values. If not for Harry, Neville could have easily stepped into the role of the main character. His growth throughout the series shows that he has the potential to be a great hero.
Voldemort could still be the main antagonist. Even though Harry has turned into a girl, Voldemort's goal of getting rid of Harry would remain the same. He might even see Harry's new form as an opportunity to weaken him, thinking that the change would make Harry less of a threat magically or in terms of leadership.
Obviously, Harry Potter himself could be a main character. He could interact with the members of One Direction, like Niall Horan. Maybe Niall is a new student at Hogwarts and Harry is assigned to show him around.
There could be a corrupt Hogwarts teacher who has been secretly practicing dark magic. This teacher, perhaps a Potions or Defense Against the Dark Arts teacher, has been influenced by the allure of power. They see Harry as a threat to their new - found power and decide to torture him in the Great Hall to send a message to the other students that they are in charge now.
Well, the plot could involve the cat - Harry Potter - stumbling into the Black Butler world. Maybe he's lost his human form due to some dark magic. He could then encounter Sebastian and Ciel. The cat - Harry - might use his magic in cat form to help or hinder the duo in their investigations. For example, he could use his invisibility ability (in cat form) to spy on enemies.