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.
Maybe a group of dark wizards who are jealous of the power of the dragon knights. They form an alliance to destroy the dragon knights and anyone associated with them, making Harry and his newfound friends their main targets.
It could be Voldemort still. Even with Harry having the power of a dragon god, Voldemort would be a major threat. He might be envious of Harry's new power and try to steal it for himself.
In some cases, it could be a jealous Hogwarts student. This student might be envious of Harry's special bond with the dragon. They could try to undermine Harry and the dragon's relationship, maybe by spreading false rumors or trying to get the dragon in trouble with the school authorities. This student could be working with some dark forces indirectly, as their envy makes them vulnerable to being influenced by the wrong side.