The Rinnegan might have the power to summon huge beasts or entities that simply overpower Konoha's defenses. In the fanfiction, it could be that the user of the Rinnegan is able to manipulate space - time in a way that disrupts Konoha's stability. For example, they could open portals that let in dangerous elements from other dimensions, like poisonous gases or aggressive creatures that ravage the village.
Well, it could be that the wielder of the Rinnegan has the ability to control powerful forces like the Six Paths techniques. They might use the Deva Path to create massive gravitational pulls that start to crumble the buildings in Konoha. Or perhaps with the Preta Path, they could drain the chakra from all the ninja in Konoha, leaving them defenseless against further attacks, which eventually leads to the destruction of the village.
It could be Sasuke. If he were to turn to the dark side in the fanfiction, his Rinnegan powers could be used to destroy Konoha. He has the potential to be extremely powerful and if he has some sort of vendetta or is influenced by some dark force, he might take his anger out on the village.
One possible reason could be a search for power. The character might feel that Konoha doesn't offer enough resources or training opportunities for them to fully explore the potential of the Rinnegan. They may believe that there are other places in the Naruto world where they can gain more knowledge and power related to their special eye.
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.
Well, it could be due to a genetic anomaly that was dormant within Naruto all along. Maybe his Uzumaki and Namikaze bloodlines combined in a unique way as he faced extreme challenges in the fanfiction, which then led to the unexpected evolution of the Rinnegan. For example, if he was exposed to a strong and strange form of chakra during a battle with an extremely powerful enemy, this could be the catalyst for such a transformation.
Some fanfictions suggest that Naruto achieves the Rinnegan as a result of intense training in a new form of chakra control. He might discover a way to tap into a deeper level of his own chakra, which has been dormant all this time. As he delves deeper into this new form of chakra manipulation, his eyes start to change. Over time, with continuous practice and facing numerous challenges that push his limits, his eyes finally transform into the Rinnegan. It could also be that he has a genetic predisposition that was activated by a specific event in the fanfiction, like coming into contact with a particular type of energy or meeting a certain character.
In some fanfictions, Naruto might awaken the Rinnegan through a powerful jutsu overload. For example, if he combines a super - charged Rasengan with some ancient and forbidden chakra - based techniques. This could potentially trigger a genetic or chakra - system shift that results in the awakening of the Rinnegan.
It might be related to a connection with some powerful entity in the Wave arc. For example, if there's a secret shinobi from the past whose power is tied to the Rinnegan and Naruto somehow forms a bond with that power source, it could lead to the awakening. Also, the intense emotions and challenges he faces in the Wave arc might be a catalyst, pushing his chakra to a new level and awakening the Rinnegan.
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.
Well, it would significantly boost his power. The Rinnegan is an extremely powerful dojutsu. Naruto already has his vast chakra reserves and Sage Mode. With the Rinnegan, he could gain new abilities like the Six Paths techniques. He might be able to manipulate gravity, use the Outer Path to revive the dead (though this has its own moral and plot implications in the fanfiction world). His combat capabilities would be off the charts, making him one of the most formidable characters in the fanfiction universe.
It could also be a situation where Naruto's connection to Hagoromo is deepened in the fanfiction. For instance, Naruto might have a vision or a spiritual encounter with Hagoromo. During this encounter, Hagoromo realizes that Naruto is the key to bringing true peace to the ninja world. So, he imparts the power of the Rinnegan to Naruto to help him achieve this grand goal. This would be a more complex and involved process, perhaps involving Naruto learning about Hagoromo's past and his own role in the grand scheme of things.