Gaara could also be part of the fanfic. Since he has his own unique abilities related to sand, there could be an interesting interaction between his powers and Naruto's summoned creatures. For example, Naruto could summon a water - based creature and they could team up with Gaara to fight an enemy that is strong against both sand and water.
One idea could be a fanfic where Naruto discovers a new and powerful summon that no one has ever seen before. This summon could have unique abilities that help Naruto in a crucial battle against a powerful enemy.
Obviously Naruto is one of the main characters. And then there are usually some of the female characters from the Naruto series like Sakura, Hinata, and maybe even some of the female ninja from other villages. They are often part of the harem that revolves around Naruto.
Sakura is another character that can be involved. Her strength and medical ninjutsu skills can be combined with other offensive or defensive jutsus. For instance, fusing her superhuman strength punch with a genjutsu to create a more complex and powerful attack. Her growth as a character throughout the series also gives room for different fusion jutsu ideas in fanfics.
Naruto himself is often a main character. Since the 'saimin no jutsu' is part of the Naruto universe, he could be either the caster or the victim of the jutsu. His unique personality and his own jutsu repertoire can interact in interesting ways with the sleeping jutsu.
Naruto himself is often involved. Since he has a strong will, it's interesting to see how he would deal with being possessed or using the possession jutsu. Sasuke is another common character. His dark past and powerful abilities make for great storylines when combined with possession jutsu.
Some fanfics might also feature Kakashi. He has knowledge of many jutsu and a past that is filled with secrets. So, he could be the one who either discovers a new aspect of a forbidden jutsu or is trying to prevent others from misusing it.
Naruto is the obvious one. His connection to the Uzumaki clan makes him the driving force behind the rebuilding. Hinata would also be likely involved, as Naruto's wife, she would support him and might contribute to the new Uzushiogakure in areas like family support and community building.
Definitely Hinata. Her gentle yet strong - willed nature could be an interesting contrast when dealing with the often - mischievous yokai from Nurarihyon. Gaara too, as his own experiences with being an outcast and having a powerful force within him could have parallels in the Nurarihyon world where some yokai are feared or misunderstood. And Rock Lee, his determination and unique fighting style could bring a new dynamic to the interactions in the fanfic.
Sasuke is a very likely character. He already has a strong connection to hate in the original series due to his clan's massacre. In a fanfic about the power of hate, his journey could be further explored, like how he might be tempted by even darker forms of hate - driven power. Another one is Obito. His fall into darkness was also driven by hate, and in a fanfic, his character could be used to show the extreme lengths that the power of hate can take someone.
Obviously Naruto himself is the central character. Sasuke would also likely be involved. Given their history and Sasuke's own complex relationship with Konoha, he might side with Naruto or at least be a key figure in the events. Kakashi too, as he has been Naruto's teacher and is an important figure in Konoha. He could be caught in the middle, trying to understand Naruto's motives and also trying to uphold Konoha's stability.