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.
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.
Sasuke Uchiha is often a popular choice. Given his natural Uchiha abilities and his connection to the Rinnegan in the original Naruto story, he's a likely candidate in fanfiction. His brooding nature and quest for power make him a great character to explore further with the added power of the Eternal Rinnegan.
The idea of new alliances and rivalries can also be explored. With the power of the Eternal Rinnegan, other ninja villages may either seek an alliance or become rivals. Fanfiction could delve into the politics and power dynamics between villages, and how the character with the Eternal Rinnegan navigates these complex relationships.
Some fanfictions depict the Eternal Mangekyou Rinnegan as having the ability to read and even rewrite the memories of others. It could also potentially have a connection to the very essence of life and death, allowing the wielder to revive the recently deceased or even curse someone with an inescapable death. Additionally, it might be able to create powerful illusions that are indistinguishable from reality, trapping opponents in a false world where they are completely at the mercy of the wielder.
In some fanfictions, they might explore the emotional and psychological impact on the character who has the eternal mangekyou rinnegan. For example, how does the power change their relationships with other characters? Do they become more isolated because of the great power they hold or do they attract more allies? It could also show how they struggle to control the power and not be consumed by it.
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 could be an original character created specifically for this fanfic. This character might be the one who actually has the eternal rinnegan and is central to the story. Their backstory, motives, and relationships with other Naruto characters would drive the plot forward.
The character often experiences a significant boost in confidence which can be both a good and a bad thing. On one hand, they are more self - assured in battles and decision - making. However, it can also lead to overconfidence. In terms of their ninja skills, they have to relearn and adapt. They may develop new fighting styles that incorporate the powers of the Eternal Mangekyou Rinnegan. This process of adaptation and the internal struggle with the power can be a major part of their character development. For instance, they might have to learn to control the urge to use the more destructive abilities of the Rinnegan, which can shape their moral compass and overall personality.
Madara Uchiha is another character who could be a main focus. Given his connection to the Uchiha clan and his own powerful Sharingan - related abilities, fanfiction could expand on his story by giving him the eternal mangekyou rinnegan and exploring what he would do with this even greater power.