Well, if Naruto got the Rinnegan as a kid in fanfiction, it would completely rewrite the story. He'd have an edge in combat right from the start. With the Rinnegan, he could see chakra in more detail, which would make his learning of new jutsu easier. His opponents would be caught off - guard by his sudden access to such a powerful dojutsu. This could lead to some interesting plot twists, like him being able to solve problems in the village or against other ninja villages more quickly and efficiently.
If a DC character got Naruto's Rinnegan in fanfiction, it would completely change their power dynamic. Take Batman for example. He's a highly skilled detective and martial artist, but with the Rinnegan, he could gain the ability to see the chakra or energy of his enemies. This would give him an edge in combat and investigations that he didn't have before.
Naruto's character would develop in multiple ways if he got the Rinnegan. Firstly, he would become more cautious. The Rinnegan is a powerful and dangerous ability, and one wrong move could have disastrous consequences. So he would think twice before acting. Secondly, he would gain a new sense of purpose. He might see himself as a guardian of a greater balance in the ninja world, not just the Hidden Leaf. His leadership skills would also be enhanced. He could use the Rinnegan to analyze battle situations more accurately and lead his comrades more effectively. Additionally, his view of the ninja world's history would expand, as the Rinnegan has connections to ancient powers and events. This would make him more of a sage - like figure, guiding the younger generation with his new - found wisdom and power.
In the wider ninja world, his relationship with other villages would change. Some villages might see him as an even greater threat now that he has the Rinnegan, and might try to form alliances against him. However, Naruto being Naruto, he would use his new power to show that he still has the same kind - hearted goals. He would try to use the Rinnegan to bring about peace even more strongly, and over time, win over those who were initially suspicious.
Naruto's character development would take a whole new turn. As a kid with the Rinnegan, he'd be seen as a prodigy. This would attract different kinds of attention, both positive and negative. He might develop a more complex view of the ninja world earlier. He would no longer be just the underdog trying to prove himself. Instead, he'd be a force to be reckoned with. His relationships with Sasuke and Sakura would also be affected. He might become more of a leader figure among them, guiding them with his newfound power, and his journey to become Hokage would have a different flavor as he would already have such a significant power advantage.
One implication is that he becomes much more powerful. The Rinnegan gives him access to powerful abilities like the Six Paths techniques, which would change the power dynamics in battles in the fanfiction. He could now face stronger enemies more easily.
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.
If Naruto got the Rinnegan, he might become more cautious. The power of the Rinnegan is so great that he would realize he needs to be more careful with it. He wouldn't be as brash as before.
In Naruto fanfiction, the Rinnegan in Naruto is often portrayed in really cool and powerful ways. Some fanfics might give Naruto the ability to use all the Rinnegan powers to their fullest extent right away, like being able to effortlessly summon the Gedo Statue or use the Deva Path powers without any drawbacks. It's a way for writers to explore new storylines and make Naruto even more of a badass than he already was in the original series.
Well, at first it might cause some strain. His sister may feel overshadowed by Naruto's new power. But as time goes on, Naruto could use the Rinnegan to help his sister discover her own latent abilities. This would transform their relationship into one of mutual growth and support. He could use the powers of the Rinnegan to train her in unique ways, and she could in turn offer him emotional support, making their bond even deeper than before.