One idea could be exploring what would happen if Harry Potter used his magic to create a parallel Hogwarts where students from different magical cultures around the world come together. This could lead to new friendships, rivalries, and the discovery of new spells and magical abilities unique to each culture.
Perhaps in this fanfiction, Harry creates a world where magic has no limits. This would mean that wizards and witches could do things like fly without broomsticks or create entire cities out of thin air. But this newfound power also attracts dark wizards who want to use it for their own evil purposes, and Harry has to protect his new world from them.
A possible plot for this fanfiction could be traveling through the different dimensions of the Harry Potter universe. There might be a dimension where Voldemort won, and seeing the desolate and dark state of the wizarding world there. Another aspect could be traveling with a group of friends to explore the magical ley lines of the world. These ley lines could be the source of powerful magic and hold secrets waiting to be uncovered. And, a journey to the magical libraries around the world, like the restricted section of the Hogwarts library on a grand scale, seeking rare and dangerous books of magic.
One idea could be being reborn as a muggle - born wizard who has a unique ability to communicate with magical creatures in ways others can't. This could lead to all sorts of adventures in the Forbidden Forest and uncovering secrets about the creatures that help in the fight against Voldemort.
One idea could be that Harry discovers a new and ancient magic that is the key to defeating a previously unknown dark force. For example, he might find a hidden spell in a long - forgotten book in the Room of Requirement.
Maybe in this fanfiction, Harry discovers a new kind of magic that allows him to reshape the wizarding world's laws across the globe. He could make Hogwarts - style education available everywhere, even in the remotest Muggle villages. This would not only spread magic knowledge but also bring the two worlds closer together.
Another idea is a story where a spell goes wrong during a Defence Against the Dark Arts class. Instead of just a minor mishap, it creates a portal to an alternate dimension where Hogwarts has a completely different layout and the magic there works in strange ways. The students then have to find their way back while dealing with the strange new magical threats in that dimension.
One idea could be exploring the wizarding world in a different time period, like the Middle Ages. Wizards could have different magical practices and face different threats. Another could be a story where Harry's children discover a hidden part of Hogwarts that has been locked away for centuries, filled with ancient magic. Maybe also a fanfiction where the roles are reversed, and the Death Eaters are the ones in power, and a group of young witches and wizards rise up against them in a new and unique way.
One idea could be that Harry creates a school for magical creatures. He realizes that there are so many unique and often misunderstood creatures in the wizarding world, and he wants to give them a place to learn and grow. This school could teach them how to interact with wizards in a positive way, as well as develop their own magical abilities.
Another plot could revolve around a conspiracy to fix the Quidditch World Cup results. A group of dark wizards might try to influence the outcome for their own gain, like betting on a certain team to lose. A young wizard or witch who discovers this plot then has to find a way to expose it and save the integrity of the tournament.
Perhaps in this fanfiction, the magic in the Harry Potter world starts to fade because of the influence of the 'Brave New World' ideology. Wizards are made to believe that their magic is a form of chaos that needs to be controlled. Harry Potter, being the hero, goes on a quest to find the source of this change and discovers that Voldemort is somehow involved with the people from 'A Brave New World'. He then has to battle not only Voldemort but also the new order that is trying to reshape the wizarding 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.