Urahara Kisuke would be a great addition. His mysterious nature and vast knowledge could either help or hinder Naruto. Yoruichi could also be a main character. Her cat - like abilities and combat skills would make for some exciting interactions with Naruto. She could teach Naruto some new moves or they could team up against a common enemy.
Well, it could be about Naruto being transported to the Soul Society and using his ninja skills there. Maybe he becomes a captain by showing his amazing combat abilities and unique jutsu. He might also interact with the existing Soul Reapers, teaching them some of his ninja ways while also learning about their powers.
There could be Sasuke. Since Naruto's situation changes, Sasuke might play a different role in this fanfic, either as an ally or a rival in a new way. Another might be Kakashi. He could be the one guiding Naruto through his discovery of being the son of chaos.
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.
Samul, of course, since he's Naruto's supposed father in this fanfic. He could be a powerful ninja who had some great influence in the past. Another character could be Naruto's mother, who might have a different story now considering her relationship with Samul. Also, characters like Jiraiya could still play an important role. He might be the one to guide Naruto in understanding his new identity as Samul's son and help him train to face the challenges that come with it.
The orphans themselves would be main characters. Each one could have their own backstory and personality. Some might be shy, while others could be more outgoing. They would be the ones directly affected by Naruto's care and teachings.
It's also possible that some of Salem's minions play important roles. They could be constantly testing Naruto's loyalty to Salem. Characters like the Grimm could also be prominent, and Naruto might have to find ways to deal with them while working for Salem.
Kushina could be an important character even though she's dead in the original story. In this fanfic, her spirit or memories could play a role in Naruto's upbringing. For example, Minato could tell Naruto stories about Kushina's feisty personality and her love for the village.
Hinata is also a likely main character. In the original story, Hinata has a crush on Naruto. With Naruto being a smooth operator, he might notice Hinata's feelings more easily and their relationship could develop in a more romantic and interesting way. Kakashi could also be involved. Naruto could use his smoothness to learn new jutsu from Kakashi more effectively or to get Kakashi to trust him with more important missions.
There could be some original Asian - inspired characters. For example, a young martial artist from a small village in China who Naruto takes under his wing. This character could be eager to learn from Naruto and would be one of the main focuses as he grows under Naruto's guidance.
A main plot point could be Naruto's arrival in Soul Society. This would be a huge shock to the system. Another could be Naruto's struggle to understand and adapt to the rules and culture of Soul Society. He might constantly butt heads with the more traditional Soul Reapers. A third plot point could be the discovery of a common enemy that threatens both Naruto's world and Soul Society. This would force them to work together, despite their differences. And finally, Naruto could use his unique skills to change Soul Society for the better, perhaps by introducing new ways of training or a different perspective on power.