The reaction would be complex. Some wizards would still have faith in Harry and believe that he can fix the situation. They would support him in his search for a cure. However, a large part of the wizarding world would be very suspicious. The Daily Prophet would be full of stories about Harry, some of which would be full of false rumors. The Hogwarts students would be divided, with some believing in their former classmate and others fearing him.
Perhaps his father's old wand. It has a connection to his family and his heritage. Since it's a wand, it already has a certain amount of magical energy, so it could potentially be a suitable vessel for a Horcrux. And it would be a very interesting twist on the concept of a Horcrux as wands are not a typical choice.
His new taijutsu could change the balance of power in battles. If it's a very powerful form of taijutsu, it could give his village an edge in conflicts. Enemies would have to find new ways to counter his moves, which might lead to a shift in military strategies across the ninja world.
One idea could be that Harry discovers an ancient spell book in the Room of Requirement that has the incantations for creating a magic railgun. He might use it to defend Hogwarts against a new, powerful dark force that is immune to traditional magic spells.
In fanfic, Harry Potter may create his own world by exploring the boundaries of magic that were previously unknown. He might stumble upon ancient spells or magical artifacts that give him the power to shape a world. He could use his connection with his friends, like Hermione's intelligence and Ron's loyalty, to help him in this endeavor. For instance, they could research together in the restricted section of the library to find ways to create new magical laws and landscapes for his world.
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.
Dumbledore, even if in spirit or through some magical means, could be an ally. His wisdom and foresight could guide Harry in making important decisions for the kingdom. Also, the house - elves, if Harry treats them fairly and gives them freedom and a place in the kingdom, could be loyal allies. They could take care of the day - to - day running of the castle or important buildings in the kingdom, using their unique magical abilities.
One key element is his magic skills. His ability to perform complex spells and charms would be crucial in building his world. Another is his creativity. Harry has seen a lot in the wizarding world, and he can use that as inspiration to create something new. Also, his relationships play a part. The support of his friends and allies would be necessary for him to bring his vision of a new world to fruition.