Perhaps Voldemort has a connection to an old enemy of King Arthur. Harry, in his quest to defeat Voldemort, travels back in time to the Arthurian era. He teams up with Merlin, who is intrigued by Harry's different kind of magic. Together they work on a plan to stop both Voldemort's past connection and present threat.
One idea could be that Harry Potter discovers an ancient magic artifact that transports him to the time of King Arthur. There, he uses his magic skills to help Arthur in a battle against a powerful dark sorcerer. Maybe he teaches some of the knights a few new spells.
A cool plot could be that Hogwarts was actually built on an ancient Arthurian site. Harry starts having strange visions related to King Arthur. He follows these visions and finds a hidden chamber under Hogwarts that has a link to Arthur's court. He then has to work with the ghosts of the knights and Merlin to protect both the wizarding world and the legacy of King Arthur from a new emerging threat.
One idea could be that Harry, as king, uses his magical knowledge to create a new system of governance in the wizarding world. He could establish laws that ensure fair treatment of all magical creatures, for example, giving house - elves proper wages and rights. Another plot could involve him facing challenges from traditional pure - blood families who oppose his more inclusive style of ruling.
One idea could be a fanfic where Harry Potter discovers a hidden portal in Hogwarts that leads him to the time of King Arthur. He could use his magic skills to assist Arthur in battles, perhaps using spells to defeat the enemies that Arthur's knights struggle with. For example, Harry could use the Stunning Spell to incapacitate a large group of invaders.
One plot idea could be that Harry starts having strange memories of Arthur's life. He might remember Arthur's relationships, like his bond with his family. This could lead to Harry trying to understand these memories and how they relate to his current life in the wizarding world. Another idea is that Harry discovers he has some of Arthur's skills, such as an affinity for working with magical devices like Arthur had with Muggle artifacts. It could create a whole new dynamic in his relationships with his friends and teachers as they try to figure out what's going on with him.
Perhaps a plot where the wizard king is from a different magical realm and comes to the Harry Potter universe. He has to adjust to the different magical customs here while also trying to solve a mystery that involves a powerful magical artifact that was lost long ago. This could involve him teaming up with Harry and his friends and teaching them some new magical techniques from his own realm.
One idea could be that Harry discovers he has the power to control monsters deep in the Forbidden Forest. He uses this new ability to protect Hogwarts from a dark wizard who is summoning dangerous creatures.
One idea could be that the Fairy King has a secret alliance with Dumbledore. They work together to protect a powerful magical artifact that Voldemort is also after. Another could be the Fairy King taking Harry Potter under his wing to teach him unique forms of magic not taught at Hogwarts.
One idea could be exploring Harry's journey in Azkaban in more depth, like how he copes with the dark and gloomy atmosphere. Maybe he discovers some long - forgotten secrets within the prison walls.
One interesting plot idea could be the discovery of a hidden connection between Harry Potter and the Dragon King. Maybe the Dragon King has some long - lost magic that only Harry can unlock, leading to a whole new adventure in the wizarding world.
Perhaps the wolf king has a prophecy related to Harry Potter. It could be foretold that the wolf king will either help or hinder Harry in his fight against Voldemort. This would create a whole new dynamic where Harry and his friends have to figure out how to deal with this new element in their battle. They might need to seek out the wolf king and try to understand its motives.
One idea could be that Harry discovers his connection to the demons due to an ancient family secret. Maybe his ancestors made a pact long ago. Another could be that he has to use his new 'King of Demons' powers to fight a dark wizard who is trying to control the demon realm for evil purposes. He could also face challenges within the wizarding world as they fear his newfound powers.