Another possible main character could be Shino Aburame. He is Naruto's friend and has his own unique abilities. In the context of Naruto joining the Inuzuka clan, Shino could offer a different perspective. He might not be from the Inuzuka clan but could provide advice to Naruto on how to adapt to a new clan environment. Also, his interactions with the Inuzuka clan members could add more depth to the story, for example, his bugs interacting with the Inuzuka dogs in some unexpected ways.
There could be a leader of the horde. This character might be initially skeptical of Naruto but later comes to respect him. Another character could be a young member of the horde who looks up to Naruto as a role model and wants to learn from him.
It could be that in this fanfiction, Naruto is on a mission with an Inuzuka member and they get into a difficult situation. Naruto shows extraordinary courage and loyalty during this mission. This impresses the Inuzuka clan so much that they start to consider him as one of their own. He then starts training with them, learning how to communicate better with the Inuzuka dogs and using their clan - specific jutsu, and he also becomes part of their family - like structure within the clan.
Obviously Naruto Uzumaki himself. He is the central figure of the Naruto series and with the focus on the Uzumaki clan wars, he would be right in the middle of it all, using his skills and the power of the Uzumaki bloodline.
Sasuke would likely be featured. Since he and Naruto have a deep bond, Sasuke might be involved in Naruto's decision to retire or in filling the void left by him. Also, Naruto's family, like Hinata and his kids, would be important characters. They would be part of Naruto's post - retirement life, showing how they support him or how they are affected by his new lifestyle.
There could be some of the older Uzushiogakure ninjas who survived the previous disasters. They would play an important role in guiding Naruto about the secrets of the village. And maybe some new characters could be introduced, like a young Uzushiogakure ninja who looks up to Naruto and helps him in his quest to save the village.
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.
Sasuke would most likely be involved. Since he has a strong bond with Naruto, he would sense something is wrong and try to free Naruto from Kaguya's control. Sakura might also be part of it as she cares about Naruto and would be determined to save him. Kakashi could play a role too, using his wisdom and experience to figure out a way to break Kaguya's hold on Naruto.
In some fanfictions, there could be an original character within the Hyuuga clan. This character could be someone who has a unique ability or perspective that draws Naruto's attention. For example, a young Hyuuga who has a different view on the clan's strict hierarchy and teams up with Naruto to bring about some changes within the clan while also facing external threats.
Sasuke could be involved. Since they have a complex relationship, Sasuke might either be a rival during Naruto's training, pushing him to be better, or he could end up joining Naruto in some training sessions to become stronger together.
Sasuke is an obvious choice. He has a connection to Naruto that goes beyond just friendship or rivalry. He might be the one who first discovers Naruto's predicament and sets out to find a solution. Sakura is essential too. Her skills as a medical ninja are crucial. She could be the one who patches Naruto up every time he gets injured in his fight against whatever is threatening his life. Also, characters like Jiraiya might make an appearance. Even though he's dead in the original story, in this fanfiction, he could come back as a spirit guide to help Naruto find a way to 'refuse to die', giving him advice based on his own knowledge and experiences.