He would surely interact with Lagoona Blue. Since Naruto loves water-based techniques, he might be interested in Lagoona's mermaid - like abilities. Also, he could meet Cleo de Nile. Naruto would probably be amazed by her confidence and the fact that she's a mummy. They could have some really interesting conversations about their different cultures and powers.
The Headless Horseman would be an interesting character for Naruto to interact with. Naruto would be both puzzled and intrigued by a character without a head. And then there's Ghoulia Yelps. Since Naruto is always eager to learn new things, he could learn a lot from Ghoulia's intelligence. They could exchange knowledge about different worlds, Naruto's ninja world and Monster High's monster world.
In this fanfiction, Naruto will surely interact with his friends who might be trying to break him out or at least communicate with him from the outside. Inside the prison, he'll encounter inmates with diverse personalities. There could be a wise old prisoner who imparts some valuable wisdom to Naruto. Also, there may be a group of inmates who form a sort of gang within the prison, and Naruto has to either fight them or make peace with them depending on the situation. And of course, there could be a mysterious figure who seems to know more about Naruto's presence in the prison than he himself does, and Naruto will have to figure out this person's motives and identity.
Ruby Rose for sure. She's the main character in Remnant and Naruto would likely be drawn to her energy and determination. He could also interact with Weiss Schnee. Naruto might be interested in her family's power and wealth, and Weiss could be curious about Naruto's strange abilities. Another one could be Blake Belladonna. Given her past and her more reserved nature, Naruto might try to befriend her and help her open up more.
He would likely interact with the Dragonborn. They could have an interesting exchange of skills, with Naruto showing off his Rasengan and the Dragonborn demonstrating shouts. He might also meet the various Jarls in the different holds, trying to understand their leadership styles which are different from the Hokage in Konoha.
In this fanfiction, Naruto might go to Monster High to seek some kind of special power or knowledge. He could make friends with the monster students, like Frankie Stein. Together they could face challenges like a bad guy who wants to disrupt the harmony between ninjas and monsters. Naruto would use his Rasengan while Frankie uses her electricity powers to fight the enemy.
Some of the Monster High characters could learn from the Naruto characters' discipline. Take Lagoona Blue for example, she could observe how the ninja train hard every day. And the Naruto characters might learn about the importance of being true to oneself from the Monster High students. Naruto could see how the monsters embrace their unique identities and be inspired to further promote acceptance in his own world.
In this fanfiction, Naruto could come across some of Kumo's young shinobi trainees. He could inspire them with his never - give - up attitude. And there's a chance he'll meet some of the village elders in Kumo who might have different views on how to deal with him, either seeing him as a potential asset or a liability.
Well, R2 - D2 and C - 3PO could be characters that Naruto interacts with. R2 - D2 could help Naruto understand some of the technology in the Star Wars universe with his beeps and whistles. C - 3PO could translate things for Naruto if he needs to communicate with different species. And then there's Finn. Finn could team up with Naruto, both being characters who are sort of outsiders in their respective worlds. They could fight side by side against the common enemies of the dark side forces in the Star Wars universe.
He could meet the Mad Hatter. The Mad Hatter would be an interesting character for Naruto to interact with as he's so unpredictable and has his own strange way of seeing the world.
Obviously, Koyuki herself would be a main character to interact with Naruto. She might show him around her place, introduce him to her people. Also, any of her loyal guards or advisors could interact with Naruto. They could be initially suspicious of him but then grow to trust him as they see his good intentions.
Interaction can also happen in a more peaceful way. A Monster Musume who is good at cooking could share her recipes with Naruto and his friends. This would lead to cultural exchange as Naruto and his pals introduce her to ninja food. And they could exchange stories about their respective worlds over a meal.