In a fanfiction scenario, Harry could betray a centaur. He might have befriended a centaur and learned some of their ancient magical secrets. But then, out of fear or misjudgment, he shares these secrets with the Ministry of Magic which the centaurs consider a great betrayal. Another plot could be that he betrays a kneazle. He gets tricked into believing that the kneazle is dangerous and hands it over to those who want to harm it. And yet another idea is that Harry betrays a bowtruckle by cutting down the tree it lives in for some seemingly important but ultimately wrong reason.
One idea could be exploring how the inheritance affects Harry's relationships with his friends. For example, if he inherits some special abilities from a creature, his friends might react with a mix of awe and concern.
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.
A plot could involve a magical creature that bonds Harry and Voldemort in an unexpected way. For example, a rare faerie that makes them see each other's memories. Through this, Harry discovers some long - hidden secrets about Voldemort's past that could potentially change the course of their enmity. They then have to decide what to do with this new - found knowledge.
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.
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 story where a new creature is discovered and brought to the school. The students have to learn how to take care of it and in the process, they discover new magical abilities related to that creature.
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 Harry discovers a hidden grove in the Forbidden Forest where various female magical creatures live. They are drawn to him because of his unique magic. Another could be that after a spell goes wrong, Harry is magically bound to protect a group of f creatures and in the process, they form a harem - like bond.
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.
One idea could be a story about a young wizard who discovers a new magical creature in the Forbidden Forest. Maybe it's a small, furry creature with the ability to turn invisible at will. The wizard tries to protect it from being discovered by other, less kind-hearted students or teachers.