Well, it could be a wild mix of magic and mutation. Maybe Harry Potter uses his magic to accidentally create a mutant in the world of 'sch' (whatever that is in the fanfiction context). It could lead to a whole new set of adventures as he tries to deal with the consequences of this creation, like trying to find a way to control or help the mutant.
It could be a dark wizard who wants to use the mutant for his own evil plans. He might see the mutant as a powerful tool to gain control over the wizarding world.
One possibility is that Harry Potter, using his knowledge of magic and experiences, decides to create a school for mutants. He could use spells to protect the school from the outside world and those who might fear the mutants. He might also teach the mutants how to control their powers, just as he learned to control his magic at Hogwarts.
In this fanfiction, there would likely be a lot of excitement among the wizarding community. Parents would be eager to send their children to a school founded by Harry Potter. He would probably face many challenges, like finding the right teachers. He might recruit some of his old friends, like Hermione, to help teach. There could also be problems with the Ministry of Magic, as they might be worried about Harry creating his own educational system.
The plot could start with Harry finding an ancient Jedi artifact during one of his adventures. This leads him to become curious about lightsabers. He then uses his magic knowledge to try and replicate it. Along the way, he might face opposition from some in the wizarding world who think it's too dangerous or against wizarding traditions. But Harry, being the hero he is, persists. Eventually, he creates a lightsaber and uses it to defeat a new, powerful dark wizard who has emerged, using both his wizarding skills and his new lightsaber skills.
In a fanfiction where Harry Potter creates a virus, perhaps it's a virus that affects magic itself. The virus could spread through spells and start to corrupt the very essence of magic. Harry, realizing the severity, might then have to team up with his old friends like Hermione and Ron. They would search for ancient magic or rare potions to counteract the virus before it destroys the entire magical ecosystem.
In a 'Harry Potter creates a shelter' fanfiction, perhaps Harry uses his magic to create a shelter in the Forbidden Forest. He might use spells like Protego Totalum to make it safe from dark creatures. It could be a place where he hides his friends during a dangerous time at Hogwarts, like when Voldemort's forces are on the prowl.
One possible plot could be that Harry, after defeating Voldemort, decides to create a kingdom where magic and non - magic folk can live together in harmony. He uses his knowledge of magic and the respect he has gained to build a new society with its own set of laws and governance that are fair to all. For example, he could create schools that teach both magic and Muggle subjects to promote understanding between the two worlds.
Well, if Harry Potter creates his own magic in fanfiction, it could lead to him being either celebrated or feared. On one hand, his new magic could solve many of the problems the wizarding world faces. But on the other hand, the Ministry of Magic might see it as a threat to their control. They could try to regulate or even suppress his new magical creations.
Well, perhaps Harry Potter, using his magic, accidentally creates the world of Remnant from RWBY. He might then have to deal with the new and strange creatures and powers there, while also trying to find a way to get back to his own world.
It could be a planet where magic is more powerful and less restricted. Harry, having faced so many limitations in the wizarding world he knew, would create a place where magic can be used freely for good. There could be no dark wizards or threats like Voldemort, and wizards could focus on exploring and expanding their magical abilities. For example, they could create new spells without the fear of them being used for evil.