Well, perhaps a plot where a character, let's say a Gryffindor, uses the Polyjuice Potion to become a centaur. As a centaur, they get involved in the centaurs' complex social structure and their views on the wizarding world. They could learn about the centaurs' prophecies and how they relate to Hogwarts. Or it could be that someone uses the potion to turn into a mermaid. They then explore the underwater world of Hogwarts, discover lost treasures or secrets that are hidden beneath the lake, and also learn about the mermaids' way of life and their relationship with the surface - dwelling wizards.
A really interesting plot could be that a group of students, in an attempt to solve a mystery at Hogwarts, use Polyjuice Potion to transform into various creatures. One turns into a thestral and discovers that the thestrals are guarding a long - forgotten chamber in the Forbidden Forest. Another turns into a kneazle and uses its heightened senses to track down a missing magical item. And a third turns into a phoenix and while in that form, they learn about the phoenix's ability to heal and its connection to the very essence of magic at Hogwarts.
One idea could be a student using Polyjuice Potion to turn into a house - elf to spy on the Hogwarts kitchen secrets. They might discover a hidden passage that the house - elves use for their own purposes. Another could be turning into a hippogriff to experience flight in a new way and interact with other magical creatures from their perspective. Maybe a character uses it to turn into a Blast - Ended Skrewt to find out why they are so aggressive and what their purpose is in the wizarding world.
One idea could be a character using Polyjuice Potion to switch with a Death Eater to infiltrate their ranks and gather information for the Order of Phoenix. This would lead to a lot of tense moments as they try not to be discovered.
One idea could be a situation where a group of Hogwarts students accidentally use the Polyjuice Potion and swap identities in the middle of an important exam. It would lead to chaos as they struggle to answer questions in each other's forms.
You could write a fanfic about a magical creature that can travel through time. It appears to Harry and his friends during their fifth year. This creature is being hunted by a powerful dark wizard from the future who wants to use its time - traveling abilities to change the course of wizarding history in his favor. Harry and his friends must protect the creature and figure out a way to send it back to its own time safely, all while dealing with the normal chaos of Umbridge at Hogwarts.
Well, perhaps a character uses the potion to turn into a teacher to change grades in order to get a friend out of academic trouble. They end up having to teach a class while in the teacher's form and hilarity ensues as they struggle to keep up with the subject matter. It could also lead to some self - discovery for the character about the challenges of teaching.
One interesting idea could be seeing how characters' relationships change. For example, if Harry was genderswapped, how would his friendship with Ron and Hermione be different? Maybe Hermione would be more protective in a different way. Another idea is the way the genderswapped character would handle situations like facing Voldemort. A female Harry might approach battles or magical challenges with a different set of skills or strategies.
Maybe the plot could revolve around a wizard who has a special ability to communicate with all magical creatures. As a result, different creatures from the Harry Potter universe, like centaurs, veelas, and house - elves, are attracted to him and form a harem. They then assist him in a quest to find a lost magical artifact that has the power to restore balance to the wizarding world.
A plot could be where a rare and endangered creature chooses Harry and Snape as its protectors. They must travel to different magical locations to find a safe haven for it. This would force them to interact and rely on each other's skills.
One idea could be that a character accidentally drinks Polyjuice Potion made from a pregnant person and has to deal with the sudden physical changes and the reactions of others. They might have to keep up the pretense while trying to find a way to reverse the effects, all while exploring the magic behind the potion in a clean, exciting way.
One idea could be exploring the backstory of a Dementor. Maybe it was once a wizard who dabbled in dark magic too much and was cursed to become that form. Another could be a story about a werewolf who tries to find a cure in the wizarding world outside of the Wolfsbane Potion. And also, a fanfiction about a Basilisk that has a secret connection to an ancient wizarding family.
Well, in some Harry Potter creature inheritance fanfiction, there could be an inheritance of a phoenix's power. This would mean the character could have the ability to heal others with their tears and rise from the ashes, literally or metaphorically, in difficult situations. Also, an inheritance of a centaur's wisdom might be a thing. The character could then have an in - depth knowledge of astronomy and divination, as centaurs are known for their understanding of the stars. Maybe even an inheritance of a mermaid's ability to swim extremely fast underwater and communicate with underwater creatures would make for an exciting plot.