It might be Harry himself. In some fanfiction scenarios, Harry could be tired of his old identity and all the baggage that comes with it. So, he decides to take on a new name, perhaps with the help of some magical means or a trusted friend, to start a new chapter in his life and escape the constant danger and scrutiny associated with his original name.
It could be Dumbledore. He is known for his strategic thinking and protecting Harry. Maybe he believes that a different name will keep Harry safer from Voldemort or other threats. Dumbledore has a deep understanding of the magical world and might have some secret knowledge about the power of names.
A new character introduced in the fanfiction could be the one. This character might have their own motives, like they are from a long - lost magical family that has a prophecy related to Harry. Giving him a different name could be their way of fulfilling the prophecy in a unique way.
One possible reason could be for protection. Maybe in the fanfiction, there are dark forces after Harry, and giving him a different name is a way to hide his true identity from his enemies. It's like in the original story where his mother's love protected him, but here, a new name adds an extra layer of security.
One of his friends could be a young werewolf who is struggling to control his condition. Harry, having faced his own challenges, would befriend him and help him find a way to live a more normal life in the wizarding world.
Perhaps Harry will meet some reclusive or mysterious figures in the new school. They could be ancient wizards who have been in hiding for years, or strange creatures that are able to communicate with humans. These encounters would surely add more excitement and mystery to Harry's time at the different school. He may also come across some students who have a different view on the wizarding war and Voldemort, which would make for interesting conversations and potential conflicts.
If Harry decides not to go to Hogwarts, the main antagonist could be the Ministry of Magic. They might see Harry as a threat since he's trying to practice magic on his own without proper supervision. They could send Aurors after him to stop him from causing any chaos in the Muggle world or from becoming a powerful, unregulated wizard.
It could be a mysterious stranger. Maybe someone who has been observing Harry from afar and decides that he needs a different wand for some greater purpose. This stranger might have their own motives, perhaps related to a prophecy or a hidden plan within the wizarding world.
His character could be more independent from the start. Since he doesn't have the initial connection to his parents' legacy, he has to create his own identity. He might be more cynical about family values at first, but as he grows, he could learn the true meaning of family through his new relationships.
In fanfiction, Harry with a different wand can lead to all sorts of interesting scenarios. Maybe the wand has different magical properties that he has to learn to control. For example, it could be more powerful in certain types of magic but less so in others. This would force Harry to adapt his magical techniques.
There could be a really strict but fair headmaster who has a unique way of teaching magic. He might be an old wizard with a lot of secrets.
Perhaps it would be a new character entirely. A powerful wizard who is jealous of Harry's arrival in this new world and sees him as a threat to his own power and status. This wizard could have control over a group of magical beings that are unique to this world and use them to oppose Harry at every turn.
It could be a new dark wizard that is specifically drawn to Harry because of his different Patronus. This wizard might see Harry's unique Patronus as a threat to his own plans for power or chaos in the magical world.