There could be members of the Gremory family. For example, Rias Gremory might be a key character. She could be one of those who initially neglect Naruto but later come to realize his potential. Another character could be Issei Hyodo, who might be in a position where he has to choose between the traditional Gremory way and Naruto's different approach.
One possible main character could be a servant or an ally of the Gremory family who initially follows the family's stance of neglecting Naruto. But as the story progresses, this character might see Naruto's true nature and start to side with him. Also, there could be an elder of the Gremory family who is strict about the family's reputation and is against any form of association with Naruto at the start.
A mysterious figure from the Gremory family's past could be a main character. This figure might have some connection to Naruto's situation, like perhaps they had a similar experience of being neglected in their time. There could also be a rival family's member who sees Naruto as a tool to bring down the Gremorys. They might approach Naruto, offering him a way to get back at those who neglected him, but Naruto has to decide if he wants to be part of such a scheme.
It could be that some of the neglected characters from Naruto, like Rock Lee for instance, are made main characters in this fanfiction and are paired with a Gremory character. Maybe a Gremory character who also has been somewhat overlooked in their own series. This would give a new perspective and new storylines to explore.
Sasuke could be one of the main characters. He might be initially suspicious of the neglected emperor, thinking he could be a threat to the balance of power. But as the story progresses, he could come to understand the emperor's situation. Another character could be Kakashi. He would likely be the voice of reason, trying to analyze the situation objectively and guide Naruto and the others in dealing with the emperor.
The plot might involve Naruto being shunned by the Gremory family or their associates. This could lead to Naruto having to find his own path. He could encounter other outcasts or those who also feel neglected. Together, they might form an alliance to change the perception of the Gremory family towards them or to simply make a place for themselves in this world that seems to not want them.
Hinata could be an idol trainee. She is shy at first but as she trains, she becomes more confident. Naruto could be her biggest supporter, and their relationship could develop further as they both pursue their dreams in the idol and ninja worlds respectively.
Besides Naruto, Yugito Nii could be a main character. She is a jinchuriki in Kumo, just like Naruto is in Konoha. They could bond over their shared experiences of being hosts. And there could be some of the Kumo elders who play important roles. They might be the ones who oppose Naruto's entry at first, fearing he could bring trouble, but then as they see how he can contribute to the village, their opinions change.
The main character has to be Naruto. But we can't forget about the Hokage, like Tsunade. She could be involved in changing Naruto's situation. Maybe she gives Naruto a special mission or training that helps him break free from being neglected. Then there are Naruto's friends from Team 7. Sasuke's relationship with Naruto is complex, and in this fanfic, it could be explored in the context of Naruto's new standing. Sakura, too, could be part of the story, perhaps by finally seeing Naruto in a new light and helping him gain more acceptance in the village.
Obviously Naruto would be one of the main characters as it's about him being the neglected child. Other characters from the original Naruto series like Iruka, Sasuke, or Sakura could also be main characters, depending on how they interact with Naruto in this fanfic.
There could be a new character introduced, perhaps an old hermit who is an expert in sealing. This character could have some hidden motives, like using Naruto's newfound skills for his own gain at first but then eventually becoming a true ally after seeing Naruto's pure intentions with the power of sealing.
A young African girl could be a main character. She might take Naruto under her wing, showing him around the village and teaching him about African traditions like dancing and making handicrafts. Also, a powerful African sorcerer could be in the story. He might view Naruto as an intruder at first but then realizes Naruto's good intentions and teams up with him to fight against a common enemy that threatens both their worlds.
Sakura could be a main character. She is a skilled medic - ninja. So, she might be involved in trying to figure out why the flowers are wilting. Maybe she thinks it's related to some new disease in the plant life of the Naruto world and she uses her medical knowledge to investigate.