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.
There could be an ancient evil spirit that has been awakened by the unique bond in Naruto's harem. This spirit wants to feed on their positive energy and love. It would use various illusions and attacks to try to separate the members of the harem and weaken Naruto so it can achieve its goal.
A group of traditionalists in Akeno's world could be antagonists. They might not approve of the idea of a harem or of an outsider like Naruto having so much influence. So they could try to break up the harem and drive Naruto out of their world by using political means, like spreading false rumors about Naruto and his friends.
Well, it could be about Naruto finding himself in a new world where he meets Akeno and gradually forms a harem. Maybe Naruto's charm and strength attract Akeno and other characters, and they all end up in this complex relationship filled with adventures, battles, and a lot of character development.
One likely main antagonist could be a powerful demon lord who is trying to break through the barriers between the spirit world and the human world. This demon lord would be a major threat to Naruto and his harem, and they would have to use all their combined skills to stop it.
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.
There could be an ancient evil force that has been dormant for a long time. The formation of the harem between Naruto and Moria Queen awakens this force as it senses the newfound power and unity. This antagonist would be extremely powerful and would pose a great threat to not only the harem members but also the entire world they are in, forcing them to work together to defeat it.
The Akatsuki could also be the main antagonists. Since they are after the bijuu, they would surely target Naruto once he has this special relationship with them. They might try more elaborate schemes to capture the bijuu from Naruto and his 'harem' of sorts.
Well, obviously Naruto and Akeno are the main characters. Naruto, the energetic and determined ninja from the Hidden Leaf Village. And Akeno, who has her own set of powers and a distinct personality from her own fictional universe. They are the focus of the fanfiction, and the story usually revolves around their interactions, whether it be a friendship, a partnership, or a romantic relationship.
One potential antagonist could be those within the village who oppose Naruto's relationships. They might see his harem as a threat to the traditional values of the village and try to cause trouble for Naruto and his partners.
In certain fanfictions, fallen angels may act as antagonists. They could be trying to disrupt the relationship between Akeno and Issei or simply cause chaos in the devil world where they live.
Perhaps a rogue ninja group that has a vendetta against Anko. When she forms this new family with Naruto, they target the whole family unit to get back at her. They might see Naruto as a way to get to Anko or vice versa, and constantly attack and create chaos for them.