One interesting concept could be exploring the reasons behind the abandonment of the Rinnegan in the fanfiction. Maybe it's due to some unforeseen consequences or a character's choice that goes against the typical use of the Rinnegan in the Naruto universe.
Sasuke could also be the main character. Given his connection to powerful eyes in the Naruto series, if the Rinnegan was abandoned, Sasuke's role would change significantly. He would have to re - evaluate his own power and place in the ninja world, which could make for a really interesting story with him as the main focus.
Well, a frequent theme is probably the struggle for identity. In these kinds of fanfictions, a character with the Rinnegan might be grappling with who they are now that they have this powerful dojutsu. Are they still the same person? Do they become something else entirely? Also, the theme of redemption could be present. Maybe the character has done some bad things because of the Rinnegan's power, and they are trying to make up for it. It's like they are on a journey to find themselves and regain their moral compass.
One reason could be that it's a very complex concept to write about. The Rinnegan has so many abilities and implications in the Naruto universe that it can be overwhelming for some writers. They start out with great ideas but then get bogged down in the details and can't continue. Another factor might be lack of time. Writers often have other commitments and can't devote enough time to fully develop their fanfiction ideas involving the Rinnegan.
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.
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.
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.
Well, when it comes to Naruto time travel and Rinnegan in fanfics, it's often about exploring new storylines. Maybe Naruto himself gets the Rinnegan and travels back in time to train his younger self. This would make him a much more powerful ninja at an earlier stage and could completely change the power balance in the Hidden Leaf Village and among the other ninja villages as well.
There's a fanfic where the female Naruto gets the Rinnegan and teams up with some unexpected allies. This fanfiction focuses on her journey to understand the true nature of the Rinnegan and how it can be used to bring peace to the ninja world. It's a great read as it shows her growth from a young kunoichi to a powerful force with the Rinnegan.