Perhaps the story could start with a strange epidemic that turns some wizards and witches into zombies. Harry discovers that it has something to do with an ancient and forbidden magic. He teams up with Hermione and Ron to search for a solution. Along the way, they encounter zombified versions of some of their classmates and teachers. They have to use their wits and magic skills, not only to survive but also to find the root of the problem and put an end to the zombie threat.
Maybe it would involve Harry and his friends having to fight off hordes of zombies in Hogwarts. The Forbidden Forest could be full of zombie creatures.
Well, it could be that Harry Potter discovers he has a connection to Sparda. Maybe he has some latent power that is related to Sparda's abilities. He then has to learn to control this new power while still dealing with the threats in his own wizarding world. And perhaps he uses his new - found powers to fight off dark wizards in a more badass way.
One possible plot could be that a new, extremely powerful and irresistible poison is created by a dark wizard. The main characters, like Harry, Hermione, and Ron, have to find an antidote before it spreads and kills many students at Hogwarts.
Perhaps it's a fanfic that takes the concept of Persephone's abduction in Greek mythology and applies it to the Harry Potter universe. The 'his' could be a dark wizard who kidnaps Persephone, a powerful witch, and the plot could be about her friends in Hogwarts trying to rescue her. Or it could be a more romantic take, where 'his' is a hero - type character and Persephone is in some sort of danger related to the ongoing battle between good and evil in the wizarding world, and he has to save her.
The plot could be about Harry Potter taking a nightly stroll in the Forbidden Forest. He might be feeling restless and decides to explore on his own. During the stroll, he stumbles upon a rare magical creature that he's only read about in books. This encounter could lead to a new adventure as he tries to protect the creature from other threats in the forest.
In a 'Harry Potter is a fighterback in time' fanfic, the plot could involve Harry going back in time for various reasons. It could be to save someone who died in the original timeline, like Sirius Black. He would then have to face the challenges of the past while also trying not to disrupt the future too much. He might also have to deal with the younger versions of his friends and teachers, and how they react to his knowledge from the future. And of course, he would be fighting against the threats of that time period, which could include Death Eaters or other dark wizards. He would need to use his skills and magic in new ways, adapting to the different time and its rules.
It could be about the last wizard in the Harry Potter world somehow getting involved with the Marvel universe. Maybe the wizard has to seek help from the Marvel heroes to save the wizarding world from a new and powerful dark force. Or perhaps the wizard brings a unique magic that the Marvel world has never seen before and causes a stir among the superheroes and villains alike.
Maybe in the fanfic, Harry is forced to play the role of a doctor during a wizarding war. He has to quickly learn how to heal the wounded soldiers on the battlefield. This could involve him using his knowledge of magic in innovative ways and also facing moral dilemmas about who to save first. It would be a story full of action and tough decisions.
Maybe Voldemort somehow resurrects an army of zombie Death Eaters as a last - ditch effort to take over the wizarding world. But this time, the good side has to team up with some unexpected allies, like the giants and the house - elves, to defeat this new threat. The story could explore how different magical creatures react to the zombie apocalypse and how their unique abilities can be used in the fight.
One way could be that Harry is somehow transformed into a RoboCop - like figure in the wizarding world. He could use his new robotic powers along with his magic to fight against dark wizards. For example, his robotic enhancements could give him super strength to physically combat opponents, while his magic could be used for more complex spells.
The plot could develop with Harry secretly studying dark magic texts in the restricted section of the library. He might start to have strange dreams or experiences related to the dark magic he's exploring, and he has to be very careful not to get caught by Filch or the teachers.