No. Fanfic that involves sexual content is not appropriate, especially when it is related to a popular franchise like Harry Potter which has a large young fan base. Such content goes against the family - friendly nature of the original work.
One common theme is self - discovery. When characters swap bodies, they often learn new things about themselves and the person whose body they are in. For example, they might realize their own insecurities or the strengths of others.
One common theme could be the use of magic to enhance physical abilities, like giving a character super strength or speed through some kind of spell or potion modification.
Hermione is a likely candidate. She's always getting into magical situations and her intelligence would make for an interesting take on dealing with a body - expanding slime. She could use her vast knowledge of magic to try and find a solution.
They add novelty. The idea of a body pillow in the Harry Potter universe is something different. It can be a fun and unexpected addition to the familiar world of Hogwarts and its characters.
It could potentially fall into the mystery genre if the 'hide' aspect involves some sort of mystery, like Harry Styles hiding something or being hidden. For example, it could be about a hidden identity or a secret he's keeping.
It could be a dark wizard who wants to use the living werewolf hide for some dark magic. Maybe he wants to create an army of werewolves under his control.
In some fanfictions, Harry might hide his money in a secret compartment of his trunk. He could use magic to create a hidden space where no one else can find it.
One interesting plotline could be Harry and Hermione switching bodies. Harry, now in Hermione's body, might struggle with the academic pressure she usually faces, while Hermione in Harry's body has to deal with sudden fame and the weight of being 'the chosen one'. Another could be Ron and Draco switching. Ron, in Draco's body, might see the world from a pure - blood elitist perspective for the first time and question his own beliefs. Draco in Ron's body could learn about the warmth and loyalty within the Weasley family.