One possible plot development could be that the Sage discovers new powers within himself while being neglected. He might use these powers to prove his worth to the Kiri villagers who overlooked him. For example, he could save the village from a hidden threat, like a powerful and secretive enemy group that the regular shinobi of Kiri were unable to handle.
Maybe the Sage decides to leave Kiri due to the neglect. He then embarks on a journey where he meets other outcasts from different villages. Together, they form a unique group with diverse skills. Later, they return to Kiri not for revenge but to show the villagers what they missed out on by neglecting the Sage.
The Sage could start to study ancient and forbidden jutsus in his solitude caused by neglect. As he delves deeper, he attracts the attention of some dark forces who want to use his knowledge for their evil purposes. This then leads to a conflict where the Sage has to decide whether to use his new - found knowledge to fight the dark forces or to try and make Kiri see his value first.
It could be a power - hungry shinobi within Kiri. This shinobi might see the Sage as a threat to his own rise in the village hierarchy. So, he uses his influence to keep the Sage neglected and also tries to sabotage any attempts by the Sage to gain recognition.
One possible plot development could be that Naruto's friends in Konoha realize he's missing and start a search mission. They might suspect Kumo immediately due to the history of tensions between the two villages. Meanwhile, in Kumo, Naruto could be held in a secret location. Some in Kumo might want to use him as a bargaining chip against Konoha, while others might see his potential and try to turn him into an asset for Kumo.
Well, perhaps Naruto, feeling neglected in his own village, decides to seek a new start in Kiri. He might meet new friends and allies there, like some of the unique ninja in Kiri. He could also face challenges from those who are suspicious of an outsider joining their ranks.
It could also be that the sister, in an attempt to help Naruto, gets involved with some dangerous characters or situations. Maybe she tries to find someone powerful to plead Naruto's case, but that person has some ulterior motives. This would add a lot of twists and turns to the story, as Naruto then has to not only deal with his own problems but also save his sister from the trouble she has gotten into while trying to help him.
A plot development could be that Kakashi discovers the root cause of Naruto's neglect. It might be related to some hidden secrets in the village. Then Kakashi has to fight against those who are behind Naruto's neglect while still training Naruto. Another development could be that Naruto and Kakashi form a very strong bond. Naruto then uses the skills and confidence he gained from Kakashi to face his enemies and also to try and change the village's attitude towards him.
Another aspect could be the relationship between Naruto and his sister. If she is the favored one, she might initially be oblivious to Naruto's pain. But as the story progresses, she could realize how wrong it is and try to make it up to him. This could lead to a complex family dynamic exploration in the fanfiction.
One possible plot development could be Naruto exploring his new bloodthirsty nature and how it affects his relationships with his friends. Maybe at first they are scared, but then they try to understand him. Another could be Naruto seeking revenge on his family for neglecting him, which might lead to some intense confrontations. And finally, he could find a new family among his comrades who accept him for who he is.
In the 'naruto becomes sage of the six paths fanfiction', perhaps Naruto would go on a journey to fully understand and master the powers of the Sage of the Six Paths. He could visit different hidden villages to learn about their ancient secrets related to the Six Paths. This would not only expand his knowledge but also help him form new alliances. And he may use his newfound status to bring about a new era of peace in the ninja world.
Perhaps Naruto is invited to Kiri to participate in a grand ninja tournament. In Kiri, he meets old friends who are also competing. He has to face off against some tough opponents from Kiri in the tournament. Along the way, he learns about Kiri's history and the importance of the tournament to their village. He also uses this opportunity to strengthen the bonds between Konoha and Kiri.
Well, perhaps the story could focus on Naruto's internal conflict. At 12, being a Jounin is a huge deal but being neglected means he has to deal with self - doubt. He could start to question his own abilities and his place in the village. As a result, he might seek training from unlikely sources outside the village, which would add an interesting twist to the story.