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.
Definitely. In some fanfics, Naruto uses the Rinnegan to rewrite the rules of ninjutsu. He could develop new techniques that are based on the Rinnegan's abilities, such as the ability to revive the dead in a different way than the traditional Edo Tensei. This makes him a target for those who oppose such a radical change in the ninja world.
One popular fanfic could be where Naruto gets the Rinnegan through some kind of ancient power transfer. In this fic, he might use the Rinnegan's powers in unique ways, like combining its abilities with his own Sage Mode and Nine - Tails chakra. This could lead to him being able to perform new jutsu that are a blend of different powers.
One popular one might be those that explore how Naruto's new power of the Rinnegan changes his relationships in the harem. For example, it could show how he uses the Rinnegan's abilities to protect the various characters in his harem during battles, adding a new layer of drama and action to the story.
One interesting fanfiction could be where Naruto gets the Rinnegan and uses it to bring a new era of peace in the ninja world. He might use its powers to understand the true nature of chakra better and teach others. Another could be a story where Naruto has the Rinnegan and teams up with Sasuke in a whole new way, combining their powers in unexpected and powerful combinations.
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.
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. You can find them on various fanfiction websites like FanFiction.net or Archive of Our Own. Some of these fanfictions explore unique storylines where Naruto gains the Eternal Rinnegan and how it changes his abilities and the Naruto universe as a whole.
A plot development might involve Naruto struggling to control the Rinnegan at first. Sasuke, noticing this, believes he can defeat Naruto easily with his Sharingan. But as Naruto starts to master the Rinnegan, he shows new abilities that shock Sasuke. This could lead to a situation where they have to team up against a common enemy who is after the power of both the Rinnegan and the Sharingan. Or perhaps a prophecy is revealed that states only one of them can keep their dojutsu power and they must fight to decide who keeps it, leading to a moral and power - based struggle.