One consequence could be that he becomes a hero among his peers. Everyone in Hogwarts might look up to him even more. The other students could see him as a real dragon - slayer, which gives him more popularity and respect.
Yes, there are likely fanfics out there with that plot. Fanfic writers often explore unique and creative scenarios like Harry killing a dragon. You can try searching on fanfic platforms such as Archive of Our Own or Fanfiction.net to find such stories.
It's highly unlikely that the characters in 'House of the Dragon' would actually watch fanfiction about themselves. Fanfiction exists in our world, created by fans for fans, and the characters in the show are fictional entities within their own fictional universe. They don't have access to the fan - created stories about them.
In certain 'Harry kills the dragon' fanfictions, Harry could be given a special magical weapon specifically designed to slay dragons. He might have to go through a series of trials to figure out how to use this weapon effectively. It could be that the weapon has to be powered by his own magic and emotions. Once he masters this, he is able to take on the dragon and kill it. For example, the weapon might need to be charged with his feelings of courage and determination, which are strong in Harry, and then he uses it to strike a fatal blow to the dragon.
Well, perhaps in this fanfic, Harry has some special equipment that helps him kill the dragon. He could have found a magical sword that is especially effective against dragons, and with his courage and quick reflexes, he manages to slay the dragon. It might also be that he has the help of his friends in some way, like Hermione providing some crucial knowledge about the dragon's weakness.
Well, perhaps the dragon is terrorizing a village near Hogwarts. Harry, being the brave hero, decides to take it upon himself to stop the dragon. He uses his magic skills, like the Patronus charm to disorient the dragon, and then a powerful blasting spell to finish it off.
Definitely not a true story. House of the Dragon is a work of imagination and creativity. It weaves a complex tale of power, politics, and dragons within a fictional realm.
Dumbledore could potentially be a source of help. He is extremely powerful and wise. He could have given Harry some advice or a special magical item before the encounter with the dragon that would prove crucial in Harry's ability to kill the dragon.
If Luffy kills a Celestial Dragon in fanfiction, it would likely cause a huge uproar in the One Piece world. The World Government would surely hunt him down with even more intensity. His bounty would skyrocket, and his friends and crew might be in more danger than ever as they would be associated with him.