Another character could be a mysterious stranger. This person could find Naruto on his journey away from Konoha and either become his mentor or his adversary. The stranger could have a hidden agenda related to Naruto's power or his connection to Konoha, which would add an interesting twist to the story.
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.
Sasuke could be involved. He might either try to stop Naruto from leaving, believing that they should face whatever comes together in Konoha, or he could decide to join Naruto on his journey, as they have a deep bond.
The elders of Konoha could be the main antagonists. They might be the ones who have been neglecting Naruto all along, and when he leaves, they see him as a threat to Konoha's stability.
Well, perhaps Naruto is neglected in Konoha because of his past or the way he behaves. He leaves and stumbles upon a hidden village that has different values. There, he is welcomed and trained further. As he grows stronger, Konoha realizes their mistake. Naruto has to decide whether to forgive Konoha and return or stay in his new - found home where he is respected.
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.
Sakura could be a main character. Since it's Naruto returning to Konoha, Sakura, being his friend and a strong ninja in her own right, might play a big role. Maybe she's the one who first welcomes Naruto back and they team up to face any threats.
One possible storyline could be that Naruto, feeling constantly neglected, decides to leave Konoha in search of a place where he can be truly accepted. He might stumble upon a hidden village that has its own unique ninja culture and there he begins to train in new and different ways, far from the scorn he faced in Konoha.
It could be a rogue ninja from another village who has a vendetta against Naruto. This ninja might spread false rumors about Naruto in Konoha, which eventually leads to Naruto leaving. So, this ninja would be the main antagonist as he is the root cause of Naruto's departure.
Maybe he marries an OC (original character). The author of the fanfic could create a whole new character with unique qualities that attract Naruto during his time away from Konoha.
Sasuke is likely to be an important character. His power and his relationship with Naruto could play a big part in the story. He might help Naruto in the fight against the Juubi or have some other role related to the sealing process.
It could be some of the more conservative village elders in Konoha. They might see Naruto Namikaze's unorthodox ways and growing power as a threat to their traditional way of running the village. So they create situations that make Naruto feel unwelcome and eventually lead to his departure, making them the antagonists in the story.