It could be an unknown brother that was kept secret all this time. Maybe he was hidden away by some enemy of the village and trained to be a powerful adversary for Naruto.
One possible plot is that Naruto's brother is jealous of Naruto's status as a hero. He decides to challenge Naruto to prove himself. The fight might start with some basic exchanges of punches and kicks. But as it progresses, they both start using their more advanced ninja skills. Naruto may struggle because he doesn't want to hurt his brother too much, but he also knows he has to stop his brother's dangerous actions.
One possible plot could be that Naruto is mislead into believing his family has been possessed by some dark force. So, he has to fight them to free them. For example, an evil ninja might use a genjutsu on his family members, making them act against Naruto's will.
It could be an enemy who has the power of sound. This opponent might be able to create sonic blasts with their music. Naruto would have to figure out how to counter these sound - based attacks, perhaps by using his wind - style jutsu to disrupt the sound waves.
In a fanfiction where Naruto fights his parents, it could be a really emotional and complex situation. Maybe it's a result of some sort of mind - control or misunderstanding. Naruto, being a strong - willed character, would likely hold back at first, not wanting to harm them. His parents, on the other hand, might be acting out of some forced obligation or false belief. It could lead to a battle that ultimately ends with Naruto finding a way to break whatever spell or falsehood is causing the rift, and they'd have a tearful reunion.
Perhaps Naruto's brother could be a long - lost relative from a far - flung part of the ninja world. He could have been separated from Naruto when they were young due to a war or some political intrigue in the ninja clans.
One possible reason could be jealousy. Maybe his brother has some special abilities or gets more attention that Naruto feels he should have. For example, if his brother is really good at a certain jutsu and is always praised for it while Naruto struggles, that could lead to resentment.
In such a fanfiction, the plot could be centered around a misunderstanding. Maybe Naruto's sister has been influenced by a villain who has made her believe false things about Naruto or the village. Naruto, being the hero he is, would initially try to talk to her but when she attacks, he has no choice but to fight back. It could also be a test of Naruto's growth, as his sister might be extremely powerful and he has to use all his skills and newfound powers to face her.
One common theme is loyalty. They always show loyalty towards each other, no matter what challenges they face. Another theme is growth. They grow together as shinobi, learning new skills and becoming stronger. And family love is also a big theme. Despite any differences, they love each other deeply as brothers.
In the world of Naruto fanfiction, the theme of Naruto killing his brother does exist. It might be a way for some authors to create a more edgy and different take on the Naruto story. They might rewrite the story's lore and Naruto's personality to make such a scenario possible. For example, they could create a backstory where there is some great conflict between Naruto and his brother that leads to this tragic outcome. This can also be a way to explore different power dynamics and the cost of power in the Naruto world.