Perhaps the Demon King has a connection to Voldemort. They could be in cahoots or the Demon King could be using Voldemort's rise to power as a distraction while he pursues his own plans. Harry then has to not only deal with Voldemort but also uncover the Demon King's true intentions and stop him before it's too late.
One idea could be that the Demon King somehow infiltrates Hogwarts. Maybe he disguises himself as a new professor. This would lead to a lot of mystery and danger as Harry and his friends try to figure out who he really is.
Perhaps a fanfic where Lily Potter's spirit is able to communicate with Harry Potter during his difficult times at Hogwarts. She gives him advice and guidance that helps him through his struggles, like dealing with Snape's complex feelings towards him, or facing the challenges of the Triwizard Tournament. This would add an emotional and magical element to Harry's journey.
One plot idea could be that Crookshanks discovers a hidden passage in Hogwarts that leads to an ancient wizarding library. He then leads Hermione to it, and they find a powerful spellbook that helps them in their fight against the dark forces. Another could be that Crookshanks has a secret ability to communicate with other magical creatures in the Forbidden Forest and he enlists their help to save Harry from a dangerous situation. Maybe he can talk to the centaurs and they come to the rescue when Harry is surrounded by Death Eaters.
One idea could be exploring Bellatrix's backstory before she became a Death Eater. Maybe she had a complex relationship with her family that led her down the dark path. Another could be a story where she has a change of heart, perhaps due to an unexpected encounter with a character like Harry Potter. For example, she could see some of her own past self in Harry and start to question her loyalty to Voldemort.
Maybe a fanfic where the goblins of Gringotts go on strike. Since Harry has some influence with them due to his own vault and his adventures there, he gets involved in the negotiations. Along the way, he uncovers a plot by some Death Eaters who are trying to take advantage of the chaos to steal valuable magical artifacts from the bank. It would be a mix of politics, mystery and action.
A plot could be that the Harry Potter headphones are a creation of a young, brilliant wizard who is trying to revolutionize the way magic and Muggle technology interact. The headphones start to cause chaos in Hogwarts as they interfere with the existing magic there. Some students love them and think they're the coolest thing ever, while the teachers are worried about the potential misuse. Meanwhile, there's a mystery about the true power of the headphones that the main character has to figure out before they fall into the wrong hands.
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.
A really interesting plot could be that the Beast King discovers a hidden power within himself that is related to an ancient prophecy. This power could be the key to saving the wizarding world from an impending doom. Harry Potter and his gang then help the Beast King to understand and control this power.
One plot idea could be that the Goblin King has a secret alliance with a dark wizard that Harry and his friends have to uncover. They might find out through a series of mysterious events and clues left in Gringotts. Another could be that the Goblin King decides to rebel against the wizarding world's control over goblin - made artifacts and starts a movement for goblin rights, with Harry caught in the middle as he tries to maintain peace. A third idea is that the Goblin King discovers a powerful ancient magic that could change the balance of power in the wizarding world and Harry has to either stop him from misusing it or convince him to use it for good.
One idea could be a crossover where Harry Potter characters visit the Pride Lands. For example, Hermione could use her magic to help solve some of the problems in the Lion King's world, like finding a new water source during a drought. Another idea is that Simba could be sent to Hogwarts for some reason and has to learn magic while dealing with the Hogwarts politics. Maybe the Sorting Hat has a hard time deciding which house to put him in because he has qualities of all the houses.
Perhaps the half - wolf demon has a special power that can help Harry Potter in his fight against Voldemort. For example, the demon can sense dark magic from a great distance. They meet in the Forbidden Forest and the demon offers his help to Harry in exchange for protection from the wizarding community's prejudice.