In a fanfiction like this, Naruto might use the Rinnegan to enhance his sealing. He could create new and more powerful seals. For instance, he could use the Rinnegan's ability to see chakra flow to design seals that are more precise and efficient at containing chakra. Maybe he discovers a way to use the Rinnegan to break existing seals that were thought to be unbreakable. This would open up a whole new world of possibilities in the Naruto universe, like freeing ancient beings or accessing hidden knowledge.
Naruto's character would likely become more confident. Having the Rinnegan, a powerful and rare dojutsu, would give him an edge in combat and in understanding the world around him. His skills in sealing would also make him more versatile. He might become more of a thinker, as he has to combine the two abilities effectively. For example, when facing a difficult situation, he would first analyze it with the Rinnegan and then come up with a sealing solution.
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.
There is a possibility that it was an ex - shinobi who had retired from active duty. This person had amassed a collection of seals during their career and now made a living by selling them to other ninjas like Naruto.
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.
Well, considering Naruto's own experiences and the variety of seals in the Naruto universe, he could sell communication seals. These would allow ninjas to send messages over long distances. He might also sell seals that are related to elemental jutsu, perhaps ones that enhance or modify fire, water, wind, earth or lightning techniques.
Perhaps Naruto was in a situation where he had made a deal. For example, he promised to pay for the seals in exchange for some information about a missing nin or a secret technique. He values knowledge and power, and if paying for the seals was part of getting closer to achieving his goals, he would do it.
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.