Well, it could make Naruto extremely powerful right from the start. With the Rinnegan, he might be able to master all types of jutsu more quickly. He could also have an edge in battles against powerful foes like Madara or Kaguya. Maybe he'd be able to use the six paths techniques with more ease and less training.
Sasuke might be the first to discover it. Since they are rivals and friends, he would be very attuned to Naruto's changes. Sasuke has his Sharingan which can detect power differences, so he could sense the new power of the Rinnegan within Naruto.
In fanfiction, Naruto might keep the Rinnegan a secret by avoiding using its powers in public. He could also use his shadow clones to create distractions when he feels like the Rinnegan is about to be revealed. Plus, he could tell only a very few trusted people, like Kakashi or Iruka, and ask them to keep it a secret for him.
Another possibility is that it's a result of some strange chakra experiment gone wrong (or right). Scientists in the Naruto universe are always dabbling in chakra research. Maybe an experiment meant to enhance her natural abilities instead gives her the Rinnegan, and she then has to learn to control it quickly to avoid being consumed by its power.
A 'naruto secret rinnegan fanfic' might center around the idea of a hidden Rinnegan technique. In this fanfic, the author could be exploring how a character obtains the Rinnegan in a very different and secretive way compared to what we've seen in the official Naruto story. It could also look at how the Rinnegan interacts with other unique abilities in the Naruto universe, perhaps creating new and unexpected combinations of powers.
In fanfiction, the idea of Naruto having the Sharingan and Rinnegan is often a creative and out - of - the - box concept. It could be due to some kind of powerful jutsu or a unique plot device. Maybe a powerful enemy uses a forbidden jutsu that somehow transfers the powers to Naruto. Another possibility is that in an alternate universe created by the fanfic writer, the rules of chakra and kekkei genkai are different, allowing Naruto to possess these powers.
Yes, there are likely to be 'harry has the rinnegan' fanfictions out there. The idea of combining elements from different fictional universes, like Harry Potter and Naruto (where the Rinnegan comes from), is a popular concept in fanfiction. Writers often like to experiment with giving characters from one world the powers or traits of another.
I would recommend 'Naruto's Rinnegan Adventure'. It's a story that delves deep into the implications of Naruto having the Rinnegan. It explores how it affects his relationships with his friends, his standing in the ninja world, and his own understanding of his place in the grand scheme of things. The story is well - written and keeps you hooked from start to finish.
In a fanfiction where Naruto secretly has the Rinnegan, he would become an even more formidable force. His combat abilities would skyrocket. He could potentially use the powers of the Rinnegan, like the Six Paths techniques, in addition to his existing Sage Mode and Nine - Tails chakra. This would make him a major threat to any enemy. He might be able to travel between dimensions more easily and have better control over life and death, just like Madara and Sasuke did with their Rinnegan.
One possible plot could be that Naruto discovers a hidden power within himself that transforms his Rinnegan into the Eternal Rinnegan. Then he has to learn to control this new power quickly as a new and powerful enemy emerges. He uses his new - found power to protect the village and his friends.