For Harry Potter fanfiction, consider writing about a magical creature that has been long forgotten in the wizarding world but is rediscovered by one of the Hogwarts students. This could lead to a whole new adventure. Another thought is to write about a Hogwarts - like school in another country and how its magic and traditions differ from Hogwarts. And then there's the idea of writing a story where the students form a secret club within Hogwarts to practice a new and forbidden type of magic.
One idea could be a story about what happens to the characters after the events of the original series. For example, exploring how Harry and Ginny raise their children and the new challenges they face in a post - Voldemort wizarding world. Another idea is to create a fanfiction set in an alternate universe where the sorting hat makes different decisions for the main characters. So, Hermione could be in Slytherin instead of Gryffindor, which would completely change her relationships and the storylines. Also, a story focusing on the backstories of the minor characters like Neville's parents or Luna's family would be really interesting.