In fanfiction, Naruto having the Rinnegan and being neglected by parents can lead to some really interesting character development. It could make him more self - reliant and distrustful at first. He might have to learn to harness the power of the Rinnegan on his own, without any parental guidance. This could result in him making some mistakes along the way but also growing stronger in unique ways.
One possible scenario in such a fanfiction could be that Naruto's parents are preoccupied with their shinobi duties. They might be constantly on missions for the Hidden Leaf Village, leaving Naruto at home with caretakers who don't really pay him much attention. This could lead to Naruto feeling lonely and isolated, just as he did in the original story when he thought he had no family.
Iruka could be one. He has always believed in Naruto from the start. Also, Jiraiya might be another. He was Naruto's teacher and had a deep understanding of Naruto's potential. And Kakashi could also support him. He has seen Naruto grow and might recognize his sage powers even when others don't.
One possible reason could be that they are too involved in their own shinobi duties. The world of ninjas is full of dangerous missions, and they might be constantly away on important tasks, leaving Naruto feeling neglected.
One plot twist could be that Naruto discovers his parents had a very good reason for their neglect, like they were protecting him from a greater danger. Another could be that the village elders knew about the neglect but did nothing, and Naruto has to confront them. Or perhaps Naruto finds out he has a secret power that was the cause of his parents' seeming neglect, as they were afraid of what he could do.
A possible plot is that Naruto is constantly bullied because of his neglected state and his unknown bloodline. As he grows, he starts to show signs of his bloodline powers, which attracts the attention of both friends and foes. His journey then becomes about controlling his powers and dealing with the past neglect of his parents while also facing the new challenges that come with his revealed bloodline.
Well, often in these fanfictions, the theme of Naruto being underestimated is prevalent. People in the Naruto world don't realize the extent of his sage abilities. There can also be a theme of revenge in some cases. Naruto, after being neglected, might seek to prove himself to those who didn't believe in him. Additionally, the theme of friendship can be explored in a different way. He might find new friends who recognize his sage powers when others don't.
Naruto might try to seek attention elsewhere. He could focus on his training more intently. By pouring his energy into becoming stronger, he distracts himself from the pain of parental neglect. Also, his friends could play a crucial role. Their friendship might give him the sense of family and belonging that he lacks at home.
One popular 'Naruto Sage Neglected' fanfiction might be 'The Forgotten Sage Path'. It often explores Naruto's journey if his sage abilities were overlooked in different ways. For example, it could show how he has to struggle more in battles without fully relying on his sage powers.