In this 'natsu becomes a girl fanfic', it could center around the idea that Natsu as a girl discovers a whole new side of the magical world. Since he is now a girl, he might be able to access certain areas or knowledge that were previously off - limits. He could also form new relationships with other characters who see him in a different light.
Lucy could also play a major role. She might be the one to help Natsu adjust to his new life as a girl. Maybe she gives Natsu tips on how to dress and act like a girl, and they could have deeper conversations about what it means to be a girl in their magical world.
Another possibility is that Natsu is turned into a girl by a magic spell gone wrong. As a girl, he discovers new magical abilities or a different way of using his existing fire magic. Maybe the transformation also affects his personality slightly, making him more empathetic and in tune with the feelings of others around him.
He could also face challenges within his own guild. Some of the members might tease him, while others might be overly protective. Natsu as a girl would have to navigate through these new social dynamics. Also, his appearance as a girl might attract unwanted attention from some villains who see him as an easier target, but he would prove them wrong with his strength.
Well, perhaps the story could start with Natsu having a strange encounter that begins his transformation into a dragon. Maybe he discovers some ancient magic or a hidden power within himself that is related to dragons.
Another option is a rival dragon slayer who sees Natsu's transformation as a threat to his own status. This rival could try to stop Natsu from fully becoming a dragon or try to defeat him to prove his own superiority.
Maybe it could involve Natsu having to go through some really tough trials to prove himself worthy of becoming the Fire Dragon King. For example, he might have to battle powerful enemies that are related to the dragon world's ancient secrets.
In addition to Natsu's Fairy Tail friends, there could be some dragon characters. Maybe Igneel, Natsu's adoptive father dragon, could play a role, perhaps in the form of a spirit or a memory guiding Natsu. There might also be other powerful dragons who are either allies or enemies depending on how they view Natsu's transformation into the Fire Dragon King.
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.
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.