Sasuke is likely to be involved. Given their complex relationship, Sasuke might be on the receiving end of Naruto's rage at times or be someone who tries to understand and calm him. Kakashi could also be there, as Naruto's sensei, he would surely be involved in dealing with Naruto's rage.
Iruka could be involved too. Since he was one of the first to really connect with Naruto in the original story. He might be the one to reach out to Naruto when his rage gets out of control. And Sakura, as part of Naruto's team, would be affected by Naruto's rage and might try to help him in her own way.
Sasuke is often involved. Since he and Naruto have a complex relationship, his reaction to Naruto's demon form can be a major plot point. He might be initially suspicious or even see it as a new way to compete with Naruto. Sakura could also be there. She might be worried about Naruto and try to find a way to help him control this new form.
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.
There might be characters from Iwa who oppose Naruto. For example, the Tsuchikage of Iwa would surely be involved as the leader of the village Naruto is attacking.
Gaara could be one of the characters. He has a bond with Naruto and if Naruto is against Konoha, Gaara might get involved to help Naruto or at least try to mediate. Shikamaru is another likely character. His intelligence would make him a key figure in trying to understand Naruto's motives and also in devising strategies for Konoha. Hinata might be involved as well, because of her feelings for Naruto. She might try to protect Naruto from Konoha's attacks.
Sasuke is often involved. Since he and Naruto have a complex relationship, Sasuke might react in various ways to Naruto's newfound power from the hidden bloodline. He could be jealous or see it as a new challenge to overcome in their rivalry.
Sasuke is often involved. Since Naruto and Sasuke have such a complex relationship, in this 'beyond godlike' scenario, Sasuke might be either a rival trying to match Naruto's new powers or an ally helping him control or use them. Another common character is Kakashi. He could be Naruto's mentor in understanding and managing these extreme powers.
Sakura could be involved. She could be like a healer or an advisor in the new world. Her medical skills would be very useful. Also, Sasuke might be there. He could be a sort of enforcer, using his Sharingan to detect threats. In the world of Asgore, they could form a new kind of team, with Naruto as the leader like Asgore. And then there could be characters from Asgore's world like Toriel. She could be a guide to Naruto about the culture and history of the place.
Sasuke could be involved. He might be one of the ones making fun of Naruto, in his typical aloof way. Or he could be in the background, just observing and secretly finding it a bit funny while pretending not to care. Sakura could also be there. She might scold the others for making fun of Naruto, or she could join in the teasing a little bit out of her own insecurities.