Well, perhaps Naruto accidentally awakens an ancient power that has the side effect of creating a harem of monster girls. These monster girls could each have different abilities and personalities. For example, a fox - like monster girl could be very sly and intelligent, while a bear - like one could be strong and protective. They all vie for Naruto's attention in their own ways.
Another plot could be that there is a threat to both the Naruto world and Monster High. Naruto teams up with the Monster High crew, and during their adventures, some of the female characters start to develop feelings for him. He is constantly saving them with his Rasengan and other jutsu, and his kind heart makes them fall for him one by one, creating a harem.
One interesting idea could be the unique cultural exchanges. For example, Naruto's ninja world culture interacting with the various cultures of the monster girls. Another might be the power dynamics, like how Naruto's strength compares to the abilities of the monster girls. Also, the relationships could be explored, such as how Naruto forms bonds with different monster girl characters in a harem setting.
One idea could be that Naruto discovers a hidden village full of monster girls. They are initially suspicious of him but as he shows his kind heart and powerful ninja skills, they start to fall for him. For example, a cat - like monster girl might be charmed by how he plays with the village cats.
One idea could be Naruto forming a unique bond with each member of the harem in different ways. For example, with Konan, he might discover her love for art and they could bond over creating origami together. Another could be Naruto teaching his harem members some of his ninja techniques, which would lead to interesting training sessions and character development.
Perhaps in the fanfiction, Naruto and Ayame start a mission together that leads to them getting stranded in a strange land. There, they gradually form a harem as they meet various characters who are attracted to their unique qualities and the bond between them.
One idea could be that Naruto, with his skills and determination, finds himself in the Sekirei world. He could form bonds with different Sekirei, like how he did with his friends in the Naruto world. Maybe he helps them in their battles and in the process, they all develop feelings for him.
One idea could be that Naruto, with his unique ninja skills, enters a new world where he meets Akeno and gradually forms a harem. They could go on adventures together, facing powerful enemies and Naruto's skills would be crucial in protecting everyone. For example, they might encounter a dark sorcerer who is trying to take over their land.
One plot idea could be a crossover where Naruto and Inuyasha are transported to a new world together and end up attracting a harem of unique characters. They could team up to protect these characters from a common enemy while also dealing with the romantic interests. Another idea is that a powerful magic or jutsu causes a group of female characters from both Naruto and Inuyasha universes to be drawn to Naruto and Inuyasha, creating a harem situation full of comical misunderstandings and adventures.
One possible plot idea could be about how Naruto has to deal with the various emotions and needs of the pregnant members of his harem. He might have to balance his ninja duties with taking care of them.
One idea could be that Naruto accidentally discovers a hidden jutsu that makes him irresistible to women, including Kurenai. This leads to a series of comical and heart - warming situations as he tries to deal with this new - found effect on the female ninja in the village, especially Kurenai who has her own reservations at first.