Sure, there are fanfictions like that. In these stories, the idea of Harry Potter being Naruto's cousin opens up a whole new realm of possibilities. It could lead to crossovers in terms of their adventures. Maybe Naruto would visit Hogwarts and learn about magic, or Harry could go to the ninja village and train as a ninja. The fanfiction could also play around with the cultural differences between the two worlds, like how the Japanese - inspired ninja culture of Naruto would blend with the British - influenced magic world of Harry Potter.
There are indeed fanfictions with this concept. It's a very creative idea. In such a fanfiction, they could potentially share some family traits. For example, both Harry and Naruto are orphans who face great challenges in their respective worlds. If they were cousins, perhaps there could be some hidden family magic or ninja skills that are passed down and only revealed when they meet. But it's all up to the imagination of the fanfiction writer.
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