Well, perhaps the plot could involve Natsu hiding his dragon identity from his friends in the guild. He might be worried about how they'll react if they find out. Maybe there are certain powers that come with being a dragon secretly that he has to learn to control without anyone noticing.
The fanfic could be about Natsu having a secret mission related to his dragon nature. He could be trying to find out more about his dragon heritage on the sly. For example, he might be searching for other dragons who could tell him about his past or his true potential as a dragon. And all this while, he has to keep up his normal façade in front of his guildmates.
In this fanfic, it could be that Natsu's dragon side is slowly emerging without his control. His friends start to notice some strange behaviors from him. Some might suspect that there's something different about him, but he keeps denying it. As the story progresses, he has to deal with the internal struggle of his human side and his emerging dragon self, all while trying to keep his secret in the guild.
Obviously, Natsu himself would be the central character. His best friend Happy could also be involved, maybe Happy starts to notice some of Natsu's strange dragon - related behaviors first. Gray might also be in the story, perhaps he gets into a fight with Natsu where Natsu's dragon powers accidentally show a bit more than he intended.
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.
Perhaps the story focuses on the bond between Natsu and the small dragon. The small dragon might have special powers that help Natsu in battles against villains in their magical world. It could also be about how Natsu takes care of the small dragon, teaching it things and growing together in their friendship.
Perhaps the fanfic focuses on the origin of Natsu's dragon - like roar. It could explore some hidden power within him that he didn't know about before. The story might involve him trying to control this new - found aspect of his power. And it could also have some sub - plots about his relationships with other characters who are either in awe or a bit scared of his roar.
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.
Well, perhaps the plot could involve Bilbo hiding her true gender identity in a world where being a girl might bring unexpected challenges or opportunities. Maybe she has to keep it a secret to be able to go on adventures like the male hobbits do. She could be constantly worried about being discovered while also having to deal with her own self - discovery and acceptance of her femininity.
In a 'lelouch is secretly a girl' fanfic, the plot could involve Lelouch's struggle with her identity. She may have been raised as a boy for strategic reasons. As she grows older, she starts to question her own identity. Meanwhile, her actions in the political and military arenas are constantly under scrutiny. When her secret is on the verge of being exposed, she has to make difficult decisions about who she really is and what she stands for. This could lead to a re - evaluation of all her relationships and her place in the world she has been fighting in.
The plot could involve Levi's past. Maybe she was forced to hide her gender to survive in a harsh environment. In the fanfic, she could gradually form a close bond with a particular character who starts to suspect something is different about her. This could lead to a conflict between her desire to keep the secret and her growing feelings for this character.
Well, one possible plot twist could be that Natsu has been hiding her true gender to be able to participate in male - dominated activities like a certain guild or sport. Another could be that she's in love with a male character, but is afraid to reveal her gender in case it changes how he feels. Or perhaps she has a special power that is related to her being a girl secretly, like a unique form of magic that only female dragonslayers can possess.
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.