There could be a powerful and jealous ninja from another village. This ninja wants the power of the Shukaku for himself. He sees the relationship between Naruto and the female Shukaku as an obstacle to his goal, so he devises plans to separate them and capture the female Shukaku, making him a major antagonist in the fanfiction.
In some fanfictions, a new character could be created as the main antagonist. This character might be jealous of Naruto and female Sasuke's growing relationship and power. They try to create rifts between them and stop them from achieving their goals.
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.
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.
The other Jinchuurikis could be potential allies. They understand the struggles and the power that comes with having a tailed beast inside them. For example, Killer Bee, who has a good relationship with his tailed beast, could share his insights on how to better co - exist with Shukaku. Tsunade might also be an ally, as she is a powerful Hokage who could offer protection and resources to the Jinchuuriki.
One possible main antagonist could be a group of rogue ninjas who are targeting Kumo. They see Naruto joining Kumo as an opportunity to cause chaos. They might try to frame Naruto for various crimes in Kumo to turn the villagers against him and Yugito, who is defending him. Another could be a power - hungry Kumo ninja who doesn't like the idea of an outsider like Naruto coming in and potentially getting more attention or power than they have.
One main antagonist could be a rogue ninja from a hidden village. This ninja has a vendetta against Konoha and targets Naruto and Hinata specifically because of their importance to the village. He uses powerful dark jutsu to try and stop them in their adventures.
One possible antagonist could be some of the traditionalist ninja in the village who oppose the marriage. They might think that Naruto, as a jinchuriki, is not a suitable match for Tenten. They could try to sabotage their relationship by spreading false rumors or creating difficult situations for them during their missions.
Maybe a powerful vampire from Moka's world who sees Naruto as an intruder and wants to get rid of him. This vampire might also be jealous of the bond between Moka and Naruto and tries to break them apart by creating chaos and fighting against them.
A group of female ninjas who oppose Naruto's presence in their world could be antagonists. They might believe that his arrival will disrupt their way of life and power structures. For example, a powerful all - female clan that has long held sway in the ninja world might see Naruto as a threat to their dominance.
The Sound Ninjas could be great antagonists. They could use their own unique parkour - like skills to try and stop Naruto. They might be chasing him through the forest, and Naruto has to use his parkour skills to outmaneuver them.