For starters, Erza would probably be more protective of Natsu as a girl. She might see her as a younger sister figure who needs more safeguarding. With Happy, their relationship could stay somewhat the same on the surface, but Happy might also be a bit more protective. Natsu's relationship with the guild master could also change. The guild master might give her different tasks or treat her more gently, which Natsu might not like at first as she's still the same tough - minded Natsu inside.
His relationship with Gray might become more complicated. Gray might be more hesitant to pick fights with Natsu now that she's a girl. And with Lucy, they could become closer as girlfriends, sharing more secrets and having sleepovers. Natsu might also attract new male friends who are interested in her as a girl.
His relationships with male friends might become more complex. They could start seeing him in a different light, maybe with a new - found attraction or a sense of awkwardness. For instance, if he had a close male friend before, that friend might struggle with how to act around him now. With female characters, he could either form a sisterly bond or face some jealousy if they see him as a threat in terms of male attention. His enemies might also underestimate him, thinking that as a girl he is weaker, but he would prove them wrong.
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 relationships with his male friends might become more complicated. Some might be more protective of him as a girl, while others might be confused about their feelings towards him. For example, if he had a rival before, that dynamic could shift completely.
Well, with Dipper becoming a girl in the fanfiction, his relationship with Mabel would have a new dynamic. Mabel would be all about embracing Dipper's new form. She'd be planning slumber parties and trying to give Dipper a new 'girly' identity. As for Wendy, Dipper might find himself more in tune with Wendy's interests on an emotional level now that he's a girl. Wendy might be more open to sharing her feelings with Dipper - girl. His relationship with the townspeople could also change. Some might be more accepting, while others might be a bit more judgmental. And his relationship with the supernatural elements in Gravity Falls could be different too. Maybe some entities are more attracted to his new form, while others are repelled, which would lead to a whole host of new adventures and challenges in their relationships.
His relationship with Naruto might change. Naruto could be more protective of Kakashi as a girl. He might see her in a different light, like an older sister figure. With Sasuke, there could be some initial awkwardness. Sasuke, who is used to Kakashi as a male, might not know how to interact with him as a girl at first. And with Sakura, they could form a closer bond as girls, sharing things that only females in the ninja world might understand.
With his female friends, there could be a shift in the dynamic as well. They might be more open with him now that he's a girl, sharing secrets and experiences that they previously wouldn't have. He could also become more involved in 'girl - only' activities, strengthening those friendships in new ways.
With his mentor or teacher figure, the dynamic would shift. Instead of being the student who is learning, Robin as a kid might be more in need of basic life lessons. His friends who were used to his equal partnership would now have to be more patient and understanding. They might find his child - like innocence endearing but also frustrating at times. And his relationship with any rivals could become more complex as they might not know how to deal with a kid version of him.
His relationship with Sasuke would likely be very different. Sasuke might be more suspicious of Naruto at first, seeing him as an unknown entity. But as they learn more about Naruto's new situation, they could develop a deeper bond based on Naruto's unique perspective.
His relationship with Hermione would be really different. As a kid, he might be more open about his feelings for her in an innocent, child - like way. He could be constantly following her around, wanting to be involved in whatever she's doing, like when they were younger in the original story but even more so. With Harry, they would probably get into more silly adventures together, like building forts in the Gryffindor common room or trying to catch magical creatures in the castle grounds.
His relationship with Naruto would likely change. Naruto might be more protective of Neji as a girl, seeing her as more vulnerable. He could also be a bit more awkward around her at first. With Hinata, they might become closer as girls and share different kinds of secrets. And his former male rivals might either be overly competitive in a different way or start to develop feelings for him, which would create a very different dynamic.