In this fanfiction, the main antagonist could be an ancient kunoichi spirit that has been awakened by Naruto's arrival. This spirit might have old - fashioned views about male and female ninja and wants to keep the world strictly for kunoichi. She could use dark magic or powerful ninja techniques to try and drive Naruto out or even harm him.
It could be a powerful kunoichi who opposes Naruto's presence in her world. Maybe she sees him as a threat to the existing power balance among kunoichi.
There could be a kunoichi who is a master of stealth and espionage. She might be the first to discover Naruto's arrival in the kunoichi world. Another main character could be a kunoichi with a strong sense of justice, similar to Naruto. She would be the one to advocate for Naruto when others are against him. And perhaps there's a kunoichi who has a special connection to an ancient power in this world, and she and Naruto's Juubi power somehow interact in an unexpected way throughout the story.
It could be a new villain who has risen to power during Naruto's absence. This villain might have a vendetta against Konoha or the ninja world in general because of something that happened while Naruto was away.
It could be a rogue ninja who has made a pact with the demons in the demon world. This ninja has turned against the ninja world and wants to use the power of the demon world to rule over all. He would constantly oppose Naruto's attempts to restore peace.
Perhaps a group of elite warriors in the new world who are jealous of Naruto's unique abilities. They form an alliance to take him down because they fear he will disrupt their status quo and take over their positions of power.
In this fanfiction, Naruto could discover that in the kunoichi world, combat styles are more focused on agility and stealth which are often associated with female ninja. He might have to adjust his own fighting style to fit in. Also, the political landscape could be shaped by kunoichi. Naruto could get involved in their power struggles, trying to bring about his own sense of justice. And he may encounter kunoichi versions of his old friends, which would be an interesting twist on his relationships.
It could be a powerful ninja who has the ability to control the monsters. He might be using the monsters to achieve his own goals, like taking over the Hidden Leaf Village.
It could be a powerful wizard in the new world who sees Naruto as a threat to his rule. Since Naruto has unique powers from his own world, the wizard might want to eliminate him or use his powers for his own evil purposes.
Definitely Sakura. Her journey from being in love with Sasuke to becoming a powerful kunoichi in her own right is always a great plot for fanfiction training. Hinata, as she tries to overcome her self - doubt and prove herself as a capable ninja. And Temari from the Sand Village can also be a focus. She's a strong kunoichi already, but in fanfiction, she can learn new skills or train with the Konoha kunoichi, which can lead to some interesting interactions.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
Sakura is really common. She has a long - standing relationship with Naruto in the original series, so it's easy to include her in a harem scenario. Hinata is also very popular. Her shy but strong - willed nature makes her an interesting addition to the harem. Tenten is another one that often shows up. She's a skilled kunoichi and can bring a different dynamic to the group.