If Natsu turns into a girl in fanfiction, his appearance would be quite a change. He might have long pink hair, and his eyes could be even more expressive. His usual dragon - slayer outfit would have to be adjusted to fit his new female form, perhaps becoming more form - fitting or having some feminine touches. In terms of his magic, it would remain the same, but the way he uses it could be different. He might be more strategic, using his new - found grace to his advantage in battles.
One possible plot could be that Natsu, after being hit by a strange magic spell during a battle, turns into a girl. His friends are initially shocked but quickly adapt. Natsu, as a girl, discovers new aspects of his own personality. For example, he might find that he is more in touch with his emotions. He could also face some challenges like having to deal with different clothing styles and the way others now perceive him. As the story progresses, he tries to find a way to break the spell while still learning important lessons about himself and his relationships with others.
One possibility is that it would bring a whole new dynamic to his relationships. His friends, like Gray, might be really confused at first. Natsu, as a girl, could potentially show a more delicate side while still having that fiery spirit. For example, in battles, she might use her strength in a more unexpected way, taking advantage of opponents underestimating her because she's a girl.
Well, Natsu's relationship with Gajeel could be really different. Gajeel might be more respectful towards Natsu as a girl, or he could be more competitive in a different way, like trying to prove that he's the better dragon slayer in a more gentlemanly manner. Natsu's relationship with the guild master, Makarov, would also change. Makarov might be more protective of Natsu as a girl, and Natsu might have to fight harder to be taken seriously in guild matters. In general, all of Natsu's relationships would be reshaped, with new levels of complexity and humor added to the story.
In some fanfictions where Natsu turns into a dragon, it often gives him extraordinary power. He might have enhanced fire abilities, being able to breathe more intense flames. His physical strength could also increase exponentially, making him an even more formidable force in battles. Maybe he would also gain some dragon - like instincts, which could either help him in combat or cause some internal conflict as he tries to balance his human side with his new dragon self.
Another theme might be how the other characters in the story react to Natsu's transformation. Their shock, acceptance, or rejection can form an important part of the plot. For example, some friends might be very supportive while others could be confused or even hostile at first.
The story would change a great deal. Natsu's usual brash and bold behavior as a boy might be tempered a bit as a girl. His interactions with the guild members would be different. For instance, the way he flirts with Lisanna might turn into a more sisterly kind of relationship if he's a girl. And his rivalry with other male characters could shift to a different type of dynamic.
There could be a powerful demon from another dimension. As Natsu becomes an adult, his magic signature becomes stronger, which attracts this demon. The demon wants to possess Natsu's power for itself and will stop at nothing to achieve this goal. It would be a formidable foe, with abilities that Natsu has never faced before.
Perhaps in this fanfiction, when Natsu becomes an adult, he decides to explore different parts of the magical world that he was previously unable to reach. He could discover new types of magic or encounter different magical creatures. This could lead to him bringing back new knowledge to his guild, and they could use it to strengthen themselves. Also, his transformation might make him more of a leader figure within the guild, guiding the younger members.
Maybe in the fanfiction, Natsu's emotional state causes him to turn into a dragon. After a great loss or a fit of extreme rage, his magic reacts and he transforms. This new form is both a blessing and a curse. It gives him the power to protect his loved ones better but also isolates him as he struggles to understand and control his new dragon instincts.