It could be a new character, an ice ninja named Hakuji. He has a tragic past, his family was killed by a rogue ninja group and he was left alone in the cold northern lands. He comes to Konoha seeking revenge at first but then befriends Naruto and changes his path.
The main character could be an existing Naruto character who discovers ice - based powers. For example, Sasuke. What if Sasuke during his travels discovers a hidden cave with an ancient ice spirit. The spirit grants him ice powers, and he has to learn to control them while still dealing with his own inner demons. This would add a whole new dimension to his character in a 'Naruto ice ninja fanfic'.
Shizune. She was Tsunade's assistant and a very capable medical ninja. A fanfic could focus on her backstory, like how she became a medical ninja and her experiences during the various battles she was a part of. She could also have her own adventures away from Tsunade.
Perhaps it could be a powerful ice spirit that has been awakened by some dark jutsu. This ice spirit has no concept of good or evil but is causing destruction as it spreads ice everywhere. Naruto and his friends would have to find a way to either seal it or make it understand the harm it's causing. In this case, the ice spirit would be the main antagonist as it opposes the well - being of the villages.
It could be an original character. For instance, a young ninja from a small, unremarkable village who has a strong sense of justice and a hidden potential. This character could be inspired by Naruto's own journey, starting from being underestimated but growing into a powerful force during the war.
It could also be a powerful and jealous ninja from another village. This ninja is envious of Naruto's growing power and popularity. He decides to create a false narrative about Naruto being a threat to all the villages and starts a war against him. Naruto would then have to prove his innocence and stop the war before it destroys the ninja world.
Maybe an entire rival village decides to target the prodigy. They see the potential threat that the prodigy poses to their own village's dominance, so they send their best ninjas to take the prodigy down.
Kakashi could also be the main character in a different take on 'naruto rogue ninja fanfiction'. Imagine if for some reason, Kakashi was framed as a rogue ninja. He would have to use his intelligence and combat skills to clear his name while also dealing with the challenges of being an outcast. He might even discover some dark secrets about the Hidden Leaf Village in the process.
It could be a group of rogue ninja who have a vendetta against Naruto. They might see his new status as a rogue as an opportunity to take him down. Maybe they were once defeated by Naruto when he was in the village, and now they want revenge. Also, there could be some hidden shinobi clans within the village who oppose Naruto's new path and try to stop him for their own reasons, like protecting the village's reputation or their own power within the village.
Arya Stark from 'Fire and Ice' could be a great main character in a 'Fire and Ice Twilight Fanfic'. She has the skills of a trained assassin and a strong will. In the fanfic, she could find herself in the world of 'Twilight' and use her skills to navigate the dangerous world of vampires and werewolves. Maybe she would form an alliance with the Cullens or the werewolf pack and help them in their battles.
It could be an original character. For instance, a ninja from a small, unknown village who has a unique power and is forced to participate in the war. They could be the underdog that everyone underestimates at first.
Typically, the shadow ninja themselves would be one of the main characters. They are often the focus, with their unique abilities and journey being central to the story. Another main character could be their mentor, someone who has trained them in the ways of the shadow ninja arts.