Hinata could be an idol trainee. She is shy at first but as she trains, she becomes more confident. Naruto could be her biggest supporter, and their relationship could develop further as they both pursue their dreams in the idol and ninja worlds respectively.
There could be members of the Gremory family. For example, Rias Gremory might be a key character. She could be one of those who initially neglect Naruto but later come to realize his potential. Another character could be Issei Hyodo, who might be in a position where he has to choose between the traditional Gremory way and Naruto's different approach.
A young African girl could be a main character. She might take Naruto under her wing, showing him around the village and teaching him about African traditions like dancing and making handicrafts. Also, a powerful African sorcerer could be in the story. He might view Naruto as an intruder at first but then realizes Naruto's good intentions and teams up with him to fight against a common enemy that threatens both their worlds.
There could be a new character introduced, perhaps an old hermit who is an expert in sealing. This character could have some hidden motives, like using Naruto's newfound skills for his own gain at first but then eventually becoming a true ally after seeing Naruto's pure intentions with the power of sealing.
Sakura could be a main character. She is a skilled medic - ninja. So, she might be involved in trying to figure out why the flowers are wilting. Maybe she thinks it's related to some new disease in the plant life of the Naruto world and she uses her medical knowledge to investigate.
Sakura could be a great main character. You could show her growth during the bell test, not just as a ninja in terms of skills, but also in her self - confidence. Maybe she has to take the lead when Naruto and Sasuke are incapacitated for some reason.
It's hard to say without reading the fanfic, but it could be characters like Sasuke. Since Sasuke is Naruto's rival, he might be involved, either competing with Naruto over the use of the Raijin no Ken or joining forces with him. Kakashi is also a likely character to be in it as he has knowledge about many ninja techniques and could play a role in guiding Naruto in using the weapon.
Perhaps there could be a storm - wizard or a shaman who adopts Naruto. This character would have deep knowledge of storm - related powers and would be the one to guide Naruto in his new journey.
Sasuke could be one of the main characters. He might be the one who comes across Naruto in his boxers and has a very typical Sasuke - like reaction, which could be a mix of annoyance and amusement. His presence could add some tension as well as some banter to the story.
One character could be Jiraiya. He could have known something about Naruto's true heritage all along and be the one to finally reveal it to Naruto. From the Greek side, Triton, Poseidon's son in mythology, could be a character. He could either be a rival or an ally to Naruto. In the ninja world, Shikamaru might be involved. His intelligence could be used to help Naruto understand and strategize with his new powers, especially when dealing with any threats that come his way because of his connection to Poseidon.
Sakura could be one. She has a long - standing relationship with Naruto and might be part of the harem. Also, Hinata. Given her feelings for Naruto, she could be drawn into this situation. Shikamaru might also be included, not in a romantic way for Naruto but as a friend who gets involved in the group dynamic. And Kiba, with his brash personality, could add an interesting element to the group.