Kakashi. His past traumas and his own brand of strength can add depth to the story. In a cold and dark setting, he can be a mentor figure to Naruto, guiding him through the difficult times while also dealing with his own inner demons.
Orochimaru. He is a character full of mystery and his dark nature fits well in a cold and dark fanfiction. He could be an antagonist constantly trying to undermine Naruto's strength or use it for his own gain.
I think Shadow could be an interesting addition. His brooding nature and his own set of powers would contrast well with Sonic's more upbeat personality. He could also have his own reasons for getting involved in the Sonic - Dark Gaia conflict. Maybe he has some past connection to Dark Gaia that no one else knows about. Also, Amy. Amy's love for Sonic can drive her to do some really brave things in the face of Dark Gaia's threat, which is always an interesting dynamic.
Another theme could be the exploration of power. Given that Naruto is strong, in a cold and dark fanfiction, his power might be a double - edged sword. It could attract unwanted attention or force him to make difficult choices. Like, he might have to use his power in ways that go against his normal moral code to survive in a harsh, unforgiving world.
Definitely Luke Skywalker. His character has so much potential for development during the Dark Empire. He could be struggling with the resurgence of the dark side and his own temptations. Another interesting character is the new Emperor. You can explore his motives and his use of the dark side power. And Leia Organa is also great to include. She could be leading the New Republic's efforts against the Empire and also dealing with her own family issues.
The Sage of Six Paths could be included. As he is the originator of many ninja powers, his connection to Amaterasu could be explored. Maybe he had a hand in creating or containing the power. His presence in the fanfiction would raise the stakes and give a more epic feel. Additionally, Hinata could be in the story. Her love for Naruto could play a role, for example, if Naruto is in danger because of Amaterasu, Hinata could try to save him in her own way.
Sasuke is a great character to include. His cool and brooding personality can create interesting dynamics in crossovers. For example, if he crosses over into the world of Attack on Titan, his skills as a ninja would be put to the test against the Titans. Another interesting character is Kakashi. His laid - back yet highly skilled nature can add humor and action to crossovers. He could be a great addition in a crossover with the world of Inuyasha, sharing his knowledge of combat and jutsu.
Well, it's interesting because it often delves deeper into the psychological aspects of the characters. For example, Naruto in these fics may face extreme betrayal or isolation, which makes us wonder how he'll cope. It also explores the darker side of the Naruto universe, like the underbelly of the ninja world that is usually not fully shown in the original series. It can be a very different and engaging take on a familiar story.
Definitely Sasuke. In fanfictions where Naruto is super strong, Sasuke often has to step up his game to keep up with Naruto. Their relationship takes on new dimensions as they both strive for greater power and different goals. Sasuke's own journey to match Naruto's strength can be really captivating.
Sasuke is always an interesting character to include. If Naruto is gender - swapped and in the Uchiha clan, Sasuke's reaction to this new Naruto would be very interesting. He might be more protective or more competitive. Itachi also plays a crucial role. His relationship with the gender - swapped Naruto could be full of mystery and depth. He could see her as a new hope for the Uchiha or be suspicious of her motives. Kakashi could also be included. His mentorship to a female Naruto in the Uchiha clan would be different. He might have different training methods or advice for her compared to the male Naruto.
One 'dark cold Naruto fanfiction' could be 'The Abandoned Shinobi'. In this fic, Naruto is shunned by the village in a much darker way than in the original story. It shows his struggle to survive and find his place in a world that seems to have turned its back on him.
Definitely Thor. Their complex relationship always adds depth to any story. You can explore their brotherly bond, rivalry, and how it changes in the context of 'The Dark World'.