In a fanfiction where Naruto is on a team by himself, his character would experience significant growth. He would start to view the world from a different perspective. Without the camaraderie of a team, he would look for other sources of motivation. His confidence could take a hit initially as he is used to having comrades. But as he overcomes challenges alone, his self - confidence would be rebuilt in a different way. He might also become more creative in his battle strategies as he doesn't have the same set of skills from teammates to fall back on. This would force him to think outside the box and develop his own unique combat approaches.
His character could develop in several ways. First, he would become more independent. He'd be forced to make all decisions on his own, which could lead to him growing up faster. Second, he might become more solitary in nature. Since he doesn't have a team to bond with, he could focus more on his own goals and desires. However, this could also make him more empathetic towards others who are alone. Third, his ninja skills would likely improve at a different pace. Without teammates to spar with regularly, he might focus more on solo training techniques and develop unique skills that are tailored to his individual fighting style.