Another option is a descendant of one of the founders of Hogwarts. They might have a special connection to dragons due to their family's ancient magic. This character could be on a quest to uncover the secrets of their family's past while also becoming a great dragon rider and facing the challenges that come with it.
Perhaps Neville Longbottom could be the main character. He's shown great courage and growth throughout the original series. In a dragon rider fanfiction, he could discover his hidden talent for dragon riding and use it to protect Hogwarts in a new and unexpected way. He already has a connection to magical creatures, so it would be a natural progression.
It could be a new student at Hogwarts. A young witch or wizard who has always been fascinated by dragons and finally gets the chance to encounter one and become a rider. This character could bring a fresh perspective to the Hogwarts world.
It could be a dark wizard who wants to control the dragons for his own evil purposes. He might see Harry as a threat because Harry is forming a bond with the dragons and teaching them to resist dark magic.
It could be an original character created specifically for this fanfiction. Maybe a young wizard or witch who has always been an outcast in the wizarding world and discovers their unique ability to connect with dark dragons.
If Harry Potter becomes a dragon rider in fanfiction, he could explore new magical places. Dragons can fly to remote areas. He might discover hidden magical lands or ancient magic that could help him in his fight against the dark forces. Maybe he'd find a new kind of magic that could defeat Voldemort once and for all.
His friends from the original story could also be main characters. For example, Hermione might be there, constantly worried about Harry's dangerous dragon - riding adventures but also helping him with research about dragons and their habitats.
One possible plot could be that there is a prophecy about a dragon rider saving the wizarding world. Harry, being the hero he is, stumbles upon a dragon that chooses him as its rider. Along the way, he has to train both himself and the dragon to fight against a new dark force that the regular magic of the wizarding world can't defeat. He might face opposition from the Ministry of Magic at first because having a dragon rider is unheard of and seen as dangerous. But as the threat grows, they come to realize that Harry and his dragon are their best hope. Harry would have to learn new spells and magic related to dragon riding and also deal with the bond between him and his dragon, which is both a source of strength and vulnerability.
A powerful dark wizard could be the main antagonist. He might be jealous of the dragon rider's powers and the bond with the dragon, and wants to break that bond and steal the rider's powers for himself.
It could be a powerful dragon hunter. This person would see Naruto's dragon as a prize to be captured or killed. They might have a personal vendetta against dragon riders in general.
A lesser - known character like Ernie Macmillan from Hufflepuff could be the main character. As a prefect, he could bring out the unique qualities of Hufflepuff house. He might have to deal with issues like other houses underestimating Hufflepuff students. His story could focus on how he promotes the values of hard work, loyalty, and fairness within the house and the school as a whole.
Hermione could be a great main character. She's curious and intelligent, so she would be really interested in exploring the secrets of the diary. She could use her vast knowledge of magic to try and figure out how the diary works and what secrets it holds.
Harry Potter himself could be the main character as a revenant. Maybe he has some unresolved issues from his past, like his relationship with his parents. His revenant form could be on a journey to fully understand their sacrifice and his own place in the wizarding world's history.