Besides Kaguya and Naruto, Sasuke is a key character that would be involved. Their rivalry - turned - friendship makes it necessary for him to step in. Then there's Kurama, the Nine - Tailed Fox inside Naruto. Kurama might be able to resist Kaguya's control to some extent and try to communicate with Naruto's consciousness. Also, characters like Shikamaru, known for his intelligence, could be involved in devising plans to break Kaguya's control over Naruto. Other shinobi from different villages might also get involved if they realize the danger that Naruto under Kaguya's control poses to the entire ninja world.
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.
Kakashi can be a popular character too. His wisdom and combat skills can be useful in fanfic scenarios involving Kaguya. He could be a mentor figure to Naruto, guiding him in dealing with Kaguya - related threats or helping to decipher ancient ninja knowledge related to Kaguya.
The two main characters would be Naruto and Kaguya for sure. However, depending on the plot, other important characters from the Naruto universe could make an appearance. It could be the tailed beasts, as Naruto has a strong connection with them. If the situation involves something related to the chakra of the ninja world, the tailed beasts might be involved in some way. Or perhaps characters from the Otsutsuki clan could be relevant if Kaguya's actions are related to her clan's history or powers.
Sasuke could be a likely candidate. He has always been Naruto's rival, and if he sees Naruto teaming up with Kaguya, who was once a great threat, he might oppose them. He may suspect that Kaguya has some ulterior motive and that Naruto is being naive in trusting her.
Sasuke can also be involved in some cases. Since he and Naruto are often together in many situations, Sasuke's presence might add some complexity to the relationship between Kaguya and Naruto. For example, Sasuke might be suspicious of Kaguya's intentions towards Naruto at first.
Obviously Naruto would be one of the main characters since it's about him being the heir of Kaguya. Sasuke could also be a main character as Naruto's rival and friend. Sakura might be involved too, as she is part of Naruto's team. And there could be other characters like Kakashi, who would play a role in guiding Naruto as he discovers his new identity. There might also be some new characters introduced that are related to Kaguya's past, who either help or hinder Naruto on his journey.
One possible plotline could be Kaguya using her powers to manipulate Naruto into fighting his friends. She might twist his perception of right and wrong, creating chaos in the ninja world. Another could be Kaguya trying to use Naruto as a vessel to regain her full power and reshape the world according to her will.
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.
Sasuke is likely to be involved. He and Naruto have a complex relationship and Sasuke has become an ally. He could offer his strength in combat to protect the rebuilding efforts. Also, Sakura. She's a skilled ninja with medical abilities that could be useful if there are any injuries during the process.
Obviously, the reincarnation of Kaguya would be a main character. Naruto or Sasuke could also be main characters as they might be involved in the discovery of the reincarnation and have to decide whether to trust her or not.