His relationship with his guildmates would likely become more complex. They might look at him with a mix of awe and a bit of fear. His closest friends would still support him, but they would have to get used to his new - found power.
Natsu's relationships would be transformed in multiple ways. With the guild, his friends like Lucy would be both proud and a little worried. They would be proud of his ascension but worried about how it might change him. His relationship with Happy would also shift. Happy might see Natsu as even more of a hero, but also be a bit intimidated. Among the dragons, Natsu would be in a position of authority. He would need to build new alliances and deal with dragon politics. This would put a strain on some of his old relationships as he has to balance his loyalty to his guild and his new role among the dragons.
His relationship with Lucy would become more complex. She might be in awe of his new power but also worried about losing the Natsu she knew. His friendships in the guild could also be strained as they might see him as a different person now. Some might be jealous of his new status, while others would be afraid that he'll leave them behind.
His friends might be initially scared. But then, they would likely be determined to help him. For example, Lucy might use her celestial magic to find a way to reverse the transformation. Their relationship would become more of a caretaker - patient one in the short term.
In a fanfiction where Natsu turns into a girl, his relationships would be turned upside down. His relationship with his friends would be filled with awkward yet endearing moments. His male friends might have a hard time adjusting at first, but gradually they would accept the new Natsu. Natsu's relationship with the female characters would also change. He might find himself being included in more girl - only activities, which would strengthen his bonds with the female members of the guild. And his relationship with the outside world, like with other guilds or villains, would also be different. Enemies might underestimate him because he is now a girl, but Natsu would prove them wrong with his strength and determination.
His relationship with Kurama might become more complex. Since he's now also a kitsune - like being. They could either become more competitive or find a new way to cooperate. With his friends, some might be in awe of his new form, while others might be a bit scared at first. But Naruto's kind heart would eventually make them all accept him as before.
If Naruto becomes a nukenin, his relationship with the village elders would be completely severed. They would see him as a threat, and he would see them as unjust. With his peers, some might side with him, believing in his innocence. Others would follow the village's orders out of loyalty. His bond with the Nine - Tails would also change. The Nine - Tails might offer more support, seeing Naruto as an outcast like itself.
His relationship with Sasuke would be greatly affected. Sasuke, who has his own experiences with the idea of killing, might see Naruto in a new light. They could either become rivals again or Sasuke might try to 'save' Naruto from his new dark path. With Sakura, she would be horrified. She's always seen Naruto as a positive force, and this change would make her question everything. She might even try to use her medical skills to 'cure' Naruto of his new - found killer tendencies.
If Naruto becomes a Saiyan in fanfiction, his relationship with his mentor, like Jiraiya, could be different. Jiraiya would have to guide Naruto in handling these new Saiyan powers. Naruto's friends might be both in awe and a little scared. For example, Sakura might be worried about Naruto losing control of his new powers. And his relationship with the villagers could change too. Some might see him as an even greater protector, while others might fear his new, more alien - like powers.
Her relationship with her family could also be different. Her parents might treat her more delicately or have different expectations. And her siblings might tease her in new ways or be more protective. Also, her relationship with her enemies could change as they might underestimate her or be more hesitant to fight a girl.
Her relationship with Natsu would likely change significantly. Since Natsu is a dragon slayer, they might become rivals in a friendly way, constantly trying to outdo each other in terms of dragon slaying skills. But they could also team up more often, combining their unique abilities to face greater threats.
Hiccup's relationship with his father (if his father is still alive in the fanfiction) would be complex. His father might be proud of him for achieving such a powerful status, but also worried about how the other tribes would react. Hiccup would also have a different relationship with the dragons in general. He would no longer be just a friend to them, but a leader, a dragon god. This could lead to some dragons being more submissive to him, while others might test his new authority.