A third theme could be self - discovery. As Natsu gets serious, he may also discover new aspects of himself. Maybe he realizes he has more power within him than he thought, or he discovers a different way of thinking about his role as a dragon slayer. This can lead to some interesting character arcs in the fanfiction.
One popular 'natsu gets serious fanfiction' could be 'Natsu's True Power Unleashed'. In this fanfic, Natsu faces a powerful dark wizard and his serious side emerges as he battles to protect his friends and the guild. It shows a more determined and strategic Natsu.
He could marry an original character created specifically for the fanfiction. This character could be a powerful mage from a far - off place who crosses paths with Natsu during one of his adventures.
One of the senior and more experienced members of the guild. They have seen a lot and know how things should be done in the guild. If Natsu acts in a way that goes against their long - established ways, they would not hesitate to berate him.
First, understand Natsu's character. Know his strengths like his determination and his fiery spirit, and his weaknesses such as his impulsiveness. Then, create a complex plot. For example, you could put him in a situation where he has to make a difficult choice between his loyalty to Fairy Tail and a greater good. This would add depth to the story.
A third theme could be facing greater challenges. In these fanfictions, the challenges are so big that Luffy can't just rely on his normal, laid - back self. He has to be more serious to overcome them.
It could be Gray. Although they are friends most of the time, in a fanfiction, there could be a plot twist where Gray's envy of Natsu's abilities gets the better of him, and he betrays Natsu. Maybe Gray thinks that by getting rid of Natsu, he can be the top mage in their guild.
Natsu might be initially shocked and confused. He's not the type to expect rejection, so he could be at a loss for words at first. For instance, he might just stand there with his mouth open, not believing what he just heard.
Well, in many fanfictions, Natsu might initially be in shock. He's a passionate character, so he could react with anger. He might lash out at Lisanna or the person she cheated with. He could also go into a state of self - doubt, wondering what he did wrong.