There are the Obelisk Blue students who might look down on Yusei at first because he's an outsider. But then he could prove himself to them. And the Ra Yellow students could be his potential friends as they might share a similar determination. He could also cross paths with the Slifer Red students and help them improve their dueling skills.
He could interact with the main characters from Duel Academy like Jaden. They might have some exciting duels together.
Yusei might meet some of the professors at the Duel Academy. For example, he could have conversations with Professor Banner, who could give him some valuable advice on dueling. Also, there could be some new students created specifically for this fanfiction that he would interact with. They could be either friends or rivals to Yusei.
Well, he might meet new friends. Maybe he'll encounter some tough rivals in duels at the academy. He could also discover new cards or strategies.
Obviously, Yusei himself. And his friends from his original time period might also show up in flashbacks or if he somehow brings them back in time with him. For example, Jack could be there.
He could interact with Tsukune, the main human character in the Yokai Academy setting. They might share their experiences of being different in their respective worlds.
Maybe Yusei goes back in time to prevent a great disaster that happened in the past. He could use his knowledge from the future to change certain events and save lives.
Crow Hogan is often a popular character to include. He's Yusei's friend and his presence can add more depth to the story. Another one could be Jack Atlas. His rivalry with Yusei and his strong personality make him a great addition. Also, Akiza Izinski might be included as she has her own powers and a connection to Yusei.
Obviously, Buffy would be a major character he'd interact with. They could start off as wary of each other but then grow to respect one another's fighting skills. Giles too, as he has a wealth of knowledge about the supernatural, and he and Harry could have some interesting discussions about different types of magic.
Sakura could have interactions with the Weasley family. Ginny Weasley might become her friend and they could have girl - talk about magic and boys. Fred and George could also be part of her story. They might play some pranks on her at first but then become good friends. And don't forget Neville Longbottom. Sakura could help Neville gain more confidence in his magic abilities as they study together in the Hogwarts library.
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.
One possible plot could be that Naruto somehow gets transported to Yusei's world. He has to adapt to the new technology and rules while still having his ninja skills. Maybe he teams up with Yusei to fight a common enemy that threatens both their worlds.
Together, they could bring out different sides of each other. Yugi might make Yusei more open - minded about the spiritual side of dueling, while Yusei could teach Yugi some practical skills like how to better maintain his cards or build a more efficient deck. Their combined traits would make for an exciting dynamic in the fanfiction.