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.
Well, it could be that Naruto, feeling neglected in his own village, decides to seek a new start in Kumo. Maybe he meets some unique characters there who accept him for who he is and he begins to train in their style of jutsu, thus becoming a stronger shinobi in a new environment.
There's also Killer Bee. Since he has a connection to the outside world and is a powerful ninja. Naruto and he could cross paths in Kiri. They might even have some rap battles or just share their ninja experiences. Another character could be Zabuza. Even though he's dead in the original story, in a fanfic, Naruto could somehow encounter his spirit or a young Zabuza - like character and learn some of his swordsmanship skills.
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.
Iruka could also be important. Since he was one of the few people who showed some kindness to Naruto in the original story. He might feel guilty about Naruto leaving due to neglect and set out to find him. His presence could add an element of emotional depth to the story as he tries to make amends.
Another possibility is that there is a rogue ninja in Kumo who uses Naruto's neglected state to his advantage. He might try to manipulate Naruto into doing things that would be bad for both Naruto and Kumo, all while remaining in the shadows and making it seem like Naruto is acting on his own accord.
Gaara would probably be one of the main characters interacting with Naruto. They both have had difficult pasts and can understand each other on a deep level. Temari and Kankuro could also be important characters. Temari might spar with Naruto, while Kankuro could share his puppet - making skills with Naruto or learn from Naruto's combat techniques.
Well, one possibility is that Naruto could quickly rise through the ranks in Kumo. Since he was neglected at home, he might be more determined to prove himself there. He could form strong bonds with new friends and comrades in Kumo who actually value him, which would be a huge change from his situation in his original village.
One possible main antagonist could be a group of rogue ninjas who are targeting Kumo. They see Naruto joining Kumo as an opportunity to cause chaos. They might try to frame Naruto for various crimes in Kumo to turn the villagers against him and Yugito, who is defending him. Another could be a power - hungry Kumo ninja who doesn't like the idea of an outsider like Naruto coming in and potentially getting more attention or power than they have.
Sasuke could be a main character as well. Given his own experiences of being an outcast and having a thirst for power at times, he could relate to Madara in some ways. They might form an unexpected alliance or have a rivalry based on their shared experiences of feeling neglected or misunderstood in the ninja world.
Sasuke could be one of the main characters. He might be initially skeptical or even hostile towards Naruto's mermaid side, but as the story unfolds, he could come to understand and support Naruto. Another character could be Sakura. She could use her medical ninjutsu skills to help Naruto if he gets injured during his underwater adventures.