Sasuke could be one of the main characters to interact with Dark Naruto. Given their complex relationship in the original Naruto series, Sasuke might be either drawn to help Dark Naruto because of their shared past, or he could see Dark Naruto as a threat to his own goals and power. They could have some intense battles or deep conversations about the nature of power and the path they've both chosen.
Sakura is another potential character. She has always cared about Naruto in the original story. In the fanfiction, she could be torn between her loyalty to the old Naruto and her fear of the new, dark Naruto. She might try to use her medical ninjutsu to find a way to heal Naruto of his dark state, or she could be a voice of reason in the chaos that Dark Naruto brings to the village.
One potential ally could be Neji. Neji has his Byakugan and his skills in combat. He could see the dark swordsman as an equal in terms of combat ability and form an alliance. Also, Tenten, who is fascinated with weapons. The dark swordsman's sword and his skills with it could draw Tenten's interest, and she might become an ally. They could exchange knowledge about different weapons and fighting techniques.
Tsunade would definitely be one. As the most renowned medical ninja in the village, she could either be Naruto's mentor or a colleague. They could exchange medical knowledge and experiences.
Naruto might interact with Eostia's warriors. They could train together, Naruto showing them his ninja combat skills and them teaching him about Eostia - style combat. There could also be some mysterious characters in Eostia, like an old hermit who has some knowledge about Naruto's sudden appearance in their world and could guide him on his journey in Eostia.
In a crossover with the Attack on Titan world, characters like Eren could interact with Naruto. Eren, with his strong will to fight for freedom, could learn from Naruto's experience as a father and a hero. Levi might also be interested in Naruto's combat skills and how he trains his child in the ways of the ninja.
He could interact with Mei Terumi, the Mizukage. She might be initially cautious of Naruto but then see his potential and work with him. Also, Chojuro, as he is one of the important ninjas in Kiri. They could train together or go on missions side by side.
Well, Kakashi could play an important role. Since he was Naruto's mentor, he would be very curious about Naruto's sudden change to a nobody. He might secretly investigate what caused this. Shikamaru could also interact with Naruto. Shikamaru is smart and observant. He might notice some small details about Naruto that others miss, and try to figure out the mystery behind Naruto becoming a nobody. Also, Rock Lee could be in the story. Lee might see Naruto as a new opportunity to spread his 'youth' spirit, not knowing Naruto's past.
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 idea could be exploring Dark Naruto's internal struggle. Maybe he's constantly fighting against the dark chakra within him and how that affects his relationships with the other characters in the Naruto universe. He could be on a journey to find a way to control or purge the darkness without losing his identity.
He could interact with Ichigo from Bleach. Since Naruto is in a godlike state, their encounter would be really intense. Ichigo might be both in awe and on guard against Naruto's overwhelming power.
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.