There could be a fanfic where Hermione accidentally makes a potion that reduces Harry and Ron to small sizes. They then use their new perspective to discover hidden secrets in the library, like tiny passages between the bookshelves that lead to forgotten chambers filled with ancient magic books.
A 'Harry Potter Small Body Fanfic' might involve Neville accidentally casting a charm on himself and his friends, making them small. As they are small, they befriend the house - elves in the kitchen in a more intimate way. The elves help them navigate the dangerous world of Hogwarts at a small scale, while they in turn help the elves with some of their problems.
One interesting 'body swap Harry Potter fanfic' could be where Harry and Draco swap bodies. It adds a whole new layer of drama as they have to navigate each other's lives. Harry has to deal with the Slytherin crowd while Draco has to face the Gryffindors. It's a fun concept that allows for a lot of character exploration.
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.
One interesting concept could be the exploration of different characters' perspectives through body shifting. For example, a fanfic might have Harry shift into Draco's body and see the Slytherin world from his eyes. It can add a whole new layer of depth to the relationships and rivalries in the Harry Potter universe.
One interesting aspect could be exploring new character developments. For example, if a girl character's body expands in a unique way, it might lead to different interactions with other characters. Maybe she gains new abilities or has to face new challenges because of this change.
You can try some fanfiction websites like FanFiction.net or Archive of Our Own (AO3). They have a large collection of Harry Potter fanfics, and you can search specifically for 'Small Body Fanfic' using their search functions.
There are fanfics where the body pillow is enchanted in some way. Maybe it has the ability to transport the characters to different places in the wizarding world. In these stories, the body pillow becomes a key element in the adventure. For instance, it could take Harry to a hidden chamber where he discovers a new spell or an ancient relic related to his parents.
There are 'Harry Potter fanfic fanfic' that focus on time - travel scenarios. Imagine if a character from the later books travels back in time to the first year at Hogwarts. This can create a lot of interesting plot twists as they try to change or preserve the original story line. Also, some fanfic fanfic might center around magical creatures in a way that wasn't fully explored in the original books, like the life of a hippogriff in more detail.
Some 'Harry Potter and Lily Potter fanfic' might focus on alternate universes. In one such story, Lily could have survived Voldemort's attack. She would then have to raise Harry in a world that is still recovering from the Dark Lord's first rise. This would change a lot of the dynamics in the story. Harry would grow up with his mother's love and protection, and Lily would have to deal with the knowledge that Voldemort could return at any time.
One popular 'harry potter body swap fanfic' is 'Swapped'. It has an interesting take on how the characters deal with being in each other's bodies. The story is full of humorous moments as they struggle to adapt to their new physical forms while still having to face the challenges in the wizarding world.
I'm not really in favor of the 'slaves' concept in fanfic as it can be a bit disturbing and against the positive spirit of the original 'Harry Potter' series. However, some fanfics might explore power dynamics in a more metaphorical way. But overall, the official 'Harry Potter' works focus on friendship, courage and magic, not slavery themes.