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.
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.
Naruto himself is the most obvious one. Since it's a Naruto - related fanfic, he would likely be at the center of any crossovers with god - like entities. His strong will and desire to protect his village make him a prime candidate for interacting with powerful gods.
Kakashi could also be involved. Maybe he's put in a difficult position where he can't save Naruto for some reason, like being under a powerful genjutsu or being blackmailed. This would create an interesting conflict as he's Naruto's teacher and mentor.
Well, characters like Jiraiya could be really interesting in a 'naruto crossover godlike fanfic'. He has a lot of knowledge about the ninja world and different powers, so his encounter with god - like beings would be full of potential for interesting stories. Tsunade too, as a powerful Hokage, could be involved. She could represent the Naruto world's power structure when dealing with these otherworldly god - like entities. And then there's Orochimaru, who is always seeking more power, so he would be very likely to get involved in any situation where there are god - like powers to be had.
Naruto and Sasuke are very likely. Their contrasting personalities and styles make a clothes swap between them full of potential. It would be a great way to show their differences and similarities in a new light.
Definitely Naruto has to be at the center. But aside from the obvious ones like Sakura and Hinata, characters like Anko Mitarashi can also be part of the story. She has a unique personality that can add a lot of spice to the harem jutsu - related plot. Also, Kurenai Yuhi could be involved. Her calm and composed nature contrasts with the chaos that the Harem Jutsu can bring. And don't forget about Karin, who has her own history with Naruto and could be an interesting addition to the harem in fanfiction.
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.
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.
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 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.