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.
He would likely use Transfiguration magic. This would allow him to shape the raw materials into the form of a staff. He could also use Enchantment spells to give the staff special powers like the ability to repel dark magic or enhance his own spell - casting abilities.
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.
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.
In some fanfictions, Harry Potter might develop his wandless magic skills through intense mental training. He could practice focusing his magic without the aid of a wand, much like how he learned to control his Patronus. By visualizing the desired outcome clearly in his mind and concentrating his magical energy, he gradually becomes proficient at wandless magic.
It means that Harry Potter's character continues to exist and develop in the world created by fanfiction writers. Fanfiction allows fans to explore different scenarios and storylines for Harry that may not be in the original books or movies.
Well, in the 'Harry Potter loses his magic fanfiction', he could end up finding new strengths within himself. Maybe he discovers that his courage and intelligence are enough to get him through tough situations even without magic. He could also start to see the wizarding world from a different perspective, one of the non - magical people, and perhaps work towards making the two worlds more inclusive.