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.
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.
Another plot could be that they are married in an alternate universe where their backstories are different. Maybe Voldemort was never as consumed by dark magic from the start. Harry and Voldemort could then discover that they have a deeper connection than they thought, and their marriage could be a journey of self - discovery for both of them, as they explore their relationship in a world that is both familiar and yet very different from the one they knew in the original story.
One idea could be that Harry starts having strange dreams where he sees through Voldemort's eyes. This could lead to him unknowingly helping Voldemort in some small ways at first, like revealing the location of some minor Order of the Phoenix safe houses. Another plot could involve the other characters slowly realizing something is wrong with Harry. For example, Hermione might notice that Harry is using some Dark Magic spells without hesitation, which is completely out of character for him.
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.
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 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.
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 that Harry travels back in time and tries to prevent Voldemort's first rise to power. He could try to stop Voldemort from creating his first Horcrux, thus changing the entire course of the wizarding war. Another plot could be Voldemort traveling to the future to see how Harry defeated him, and then using that knowledge to change his past strategies. Maybe he would try to find a way to destroy the prophecy before it was made.
Well, a possible plot could be that Harry is cursed in a way that slowly changes his values and beliefs. As he starts to side with Voldemort, he begins to enforce new, dark rules in Hogwarts. The other students are in shock and disbelief. Some of his friends like Hermione might try to break the curse, while others like Ron might be more quick to condemn him. Also, Voldemort could use Harry as a tool to gain more support from the wizarding community, as Harry was once seen as the 'Chosen One' to defeat him. So with Harry on his side, Voldemort could deceive many into thinking his ways are not so bad.