Sasuke could be a likely candidate. He has always had a bit of a lone - wolf streak in the original series. He might decide to form his own 'one man team' to pursue his own goals, like finding out more about the Uchiha clan and getting revenge in his own way without depending on others too much.
It could be Naruto himself. There might be a scenario where he feels he has to go solo for a while to protect his friends. He forms his own 'one man team' in a sense to take on threats alone so that his friends don't get hurt. This would show his selfless nature and his growth as a character who is willing to bear the burden alone.
It could be a rogue ninja from a rival village. This ninja has a personal vendetta against Konoha and specifically targets Team 10 because of Naruto's presence. He might be jealous of Naruto's growing power and fame and wants to prove himself by defeating the team.
In such fanfiction, Naruto's self - reliance grows. He becomes more confident in his abilities as he doesn't have a team to fall back on. For instance, he may start making his own battle strategies without consulting others, leading to more creative and risky approaches in combat.
Perhaps a forward - thinking businessman. He might be the one who sees the potential in new technologies and invests heavily in them. He then uses his business acumen to spread these technologies across the fictional land, making him the catalyst for the industrial revolution. For example, he could be the first to mass - produce a new type of machinery, which then becomes the cornerstone of many industries.
Mega Man Zero could also be the main character if the story focuses on his adventure in the Naruto world. He would be the outsider, having to learn about chakra, ninja clans, and the political structure of the Naruto world. His combat skills and sense of justice would play a big role in how he navigates this new and strange world.
Another option could be a new character. A young ninja from a small, unknown village who discovers the 'All For One' ability. This character would be starting from a place of relative obscurity and would have to learn how to control the power while also dealing with the attention it brings from more powerful ninja. They could be on a journey to understand the power and use it for the good of their village and the entire ninja world.
Another option could be a new character entirely. This character could be a hybrid of the two worlds, having some abilities from Naruto and some from One Piece. His unique background and skills would make him stand out, and as he tries to find his place in this combined world, he could attract a harem of female characters. He could be on a quest to bring peace between the two worlds, and during this journey, he builds relationships with various female characters.
Perhaps an old enemy of one of the members of Team 9 shows up. Since Naruto has joined the team, they might see it as an opportunity to get revenge. This antagonist could be someone who was once defeated by the member's family and now wants to prove themselves. They would be very cunning and use their knowledge of the team member's past to try and divide the team.
It could be Danzo. He's always been a bit of a shadowy figure in the Naruto series. Maybe he sees Naruto as a threat to his power and influence in the village and manipulates the situation so that Team 7 has to abandon him. He would be a great main antagonist as he has the resources and the motives to cause such a rift in Team 7.
It could be Orochimaru. He has always been interested in Sasuke and might target ANBU Team 7 to get to him. Orochimaru's experiments and his desire for power make him a formidable and menacing villain. He could also be using some new jutsu or have created more powerful minions to fight against Team 7.
Well, in such a fanfic, Naruto could be shown as a lone wolf. His character might be more solitary, focusing only on his own goals. He may not rely on the support of his friends like Sasuke or Sakura. He might develop his own unique fighting style that doesn't require teamwork. His relationships with others could also be different, perhaps more distant as he's so used to being on his own in this fictional scenario.