The main antagonists could be a group of rogue ninjas. They target Naruto because of his reputation as a banished and potentially vulnerable target. And they also see Fem Asuma and her group as an obstacle. They might try to kidnap the female characters to lure Naruto into a trap, or steal valuable resources from them to undermine their growing community.
Well, perhaps in this 'banished naruto x fem asuma harem fanfiction', Naruto is banished due to false accusations. Fem Asuma, who has her own set of skills and a strong personality, takes pity on him. The harem aspect could start when Naruto starts to train with Fem Asuma and other female trainees join in. They see Naruto's unique abilities and his kind heart, which makes them fall for him one by one.
It could be Orochimaru. Since Orochimaru is always after powerful ninja and their abilities, if Naruto is now the son of Asuma, he might see Naruto as a new target with unique potential.
In a few fanfictions, it could be a jealous ninja within the village. Maybe someone who has feelings for Naruto and sees Sakura as an obstacle. This ninja could create a false narrative that leads to Sakura's banishment, thus becoming the main antagonist in the story.
One possible antagonist could be a powerful wizard or sorcerer who is jealous of Naruto's growing influence and the harem he has. This antagonist might try to break up the group and steal their powers.
Sakura could be one. After Naruto's banishment, she might realize her true feelings for him and follow him. Hinata is also a likely candidate. Her long - standing admiration for Naruto could lead her to join him during his banishment. And then there could be some original female characters created specifically for the fanfiction, perhaps a powerful kunoichi from a far - off land who is intrigued by Naruto's story.
The main antagonists could be the ones who banished Naruto in the first place. Maybe it was a group of elders in the village who feared Naruto's power and wanted to get rid of him. They would see Naruto as a threat now that he is becoming the Onikage and would try to stop him from returning to the village or causing any changes.
It could be the ninja from Naruto's former village who don't want him to succeed in his new place. They might see his new - found influence as a threat and try to undermine him.
The main antagonists could be some of the conservative Iwa ninjas who oppose Naruto joining their village. They might see him as a threat to their traditional ways and values. Also, there could be some rogue ninjas from other villages who target Naruto because they see him as an easy target now that he's in a new and unfamiliar environment.
The main antagonists could be some of the conservative elders in the Hidden Leaf Village. They banished Naruto in the first place and would see his rebuilding of the Uzumaki clan as a threat to their power. They might try to undermine his efforts by spreading false rumors about him and his new clan.
The village elders could be the main antagonists. They might see Naruto's godlike powers as a threat to their traditional way of ruling the village and decide to get rid of him. Another option could be a secret organization within the village that wants to use Naruto's power for their own evil purposes and when he refuses, they turn on him.