There could be a group of bandits who have been causing trouble around the areas where Naruto is training Team 8. They don't like the idea of a strong team emerging in their territory. So, they start attacking the team during their training sessions. These bandits might have some crude but effective combat techniques and they also use the terrain to their advantage, which makes them a formidable antagonist for Team 8 during their training with Naruto.
It could be a rogue ninja from another village who has a grudge against Uzuki and wants to stop her from training Naruto. This ninja might be jealous of Uzuki's skills or have some past conflict with her. They could try to disrupt the training sessions by attacking Naruto or setting traps.
Perhaps the main antagonist is actually an old enemy who manipulates Naruto's team into abandoning him. This enemy could be using mind - control or false information to drive a wedge between Naruto and his team. They do this to weaken Naruto, knowing that his strength comes from his relationships.
Danzo could also be the main antagonist. He is known for his underhanded methods and distrust of anyone with too much power. Since Naruto has the power of the Nine - Tails, Danzo might spread lies within Team 7 to make them think Naruto is a danger, causing them to betray him.
It could be a former member of Akatsuki who has a personal vendetta against Naruto. Maybe they escaped from prison and now they want to take revenge on Naruto for all the things he did to Akatsuki. They would be a very powerful and dangerous antagonist as they possess knowledge of many dark jutsus.
The villagers themselves could be the antagonists in a sense. They have always been somewhat afraid and resentful of Naruto because of the Nine - Tails. Their constant negative attitude towards Naruto could influence the decision to kick him out of Team 7. They might pressure the village elders and Kakashi into believing that Naruto is not a suitable member for the team and that he poses a threat to the safety of the other team members.
It could be Frieza. Frieza might sense Gohan getting stronger under Beerus' training and see him as a threat. He decides to stop Gohan's progress by any means necessary.
In a 'naruto trains team 8 fanfic', the plot could revolve around Naruto's unique training methods. He could take Team 8 on a journey to different locations to train. For example, they could go to a forest filled with rare chakra - rich plants that enhance their abilities while training. Naruto would also share his experiences from his own training with Jiraiya, inspiring Team 8 to push their limits. He might introduce them to new forms of sparring that involve not just physical combat but also mental and chakra control challenges.
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.
The plot could involve Naruto passing on his unique skills. For example, he might teach the genin his signature Rasengan. He could also share his experiences of facing tough opponents, like Sasuke. Maybe he'd take the team on a mission to a place filled with mystery, like the Land of Waves, and teach them how to handle real - world situations.
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.