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.
One possible plot could be that Naruto, with his sense of justice, witnesses the tyranny of the Celestial Dragons. He decides to take action when they harm his friends or innocent people. He uses his ninja skills, like his shadow clones and rasengan, to fight them. In this fanfiction, Naruto might be on a journey in the One Piece world where he encounters these despicable Celestial Dragons.
After Luffy kills a Celestial Dragon in fanfiction, the story might see the emergence of new allies for the Straw Hat Pirates. There are likely many characters in the One Piece world who secretly despise the Celestial Dragons and would be inspired by Luffy's bold move. These new allies could bring new skills and resources to the crew. Also, the World Government might start to show its cracks as internal disputes could arise over how to handle Luffy. Some might want to go all - out to destroy him, while others might see the potential for a change in the current power structure.
Sakura might be initially shocked as killing is a serious matter, but then she would understand Naruto's reasons if she saw the cruelty of the Celestial Dragons. She would support him in the end. Kakashi, on the other hand, would probably be more analytical. He would first assess if Naruto's actions were truly necessary and if there were any other ways to deal with the situation. But once he realizes the nature of the Celestial Dragons, he would back Naruto up.
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.
There could be. Fanfiction writers love to take characters from one universe and place them in the context of another, or give them entirely new identities. Given Naruto's popularity, it's not unlikely that someone has written a piece where he is a Celestial Dragon. However, it might be a bit of a niche concept, so it could take some searching.
I'm not sure specifically as there could be many different 'celestial dragon fanfics'. It might be about celestial dragons, which are often a concept in fantasy or some fictional universes. They could be about their powers, their place in a certain world, or their interactions with other characters.
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.