One possible theme could be power and responsibility. Since Fairy Law is a powerful spell, Natsu using it might come with the need to be responsible for the consequences. Another theme could be friendship. His friends may be involved in the situation where he uses Fairy Law, and their support or lack of it could be explored.
Well, in a 'natsu uses fairy law fanfiction', it often showcases Natsu's power in a more creative and unique way. Fairy Law is a very powerful spell in the Fairy Tail universe, and when Natsu uses it in fanfiction, it could lead to different plot developments. Maybe he uses it to defeat a much stronger enemy than in the original story, or perhaps he uses it in a way that reveals new aspects of his character, like his growth in controlling such a powerful magic.
One possible theme could be power and responsibility. As Natsu is in a 'king' role, he has to handle the power that comes with it and the responsibilities towards his friends and the guild.
Often, these fanfictions focus on Natsu's growth in using his fairy magic. They might show him learning new spells or techniques.
One great 'natsu uses fairy magic' fanfiction is 'Fairy Tail: Natsu's New Power'. In it, Natsu discovers a new way to use his fairy magic which leads to some amazing battles against powerful foes.
One reason could be that the fanfictions don't match his view of the Fairy Tail world. He has his own understanding of the guild, his friends, and their adventures, and when fanfictions deviate from that, he doesn't like it.
One possible plot could be that Natsu discovers a magic that allows him to sing and this singing has an effect on a snow fairy. Maybe his song can heal the snow fairy or break a curse on it.
One possible storyline could be Natsu founding Fairy Tail after a great battle where he realizes the need for a guild that stands for true friendship and strength. He then gathers like - minded mages and builds the guild from the ground up.
One possible storyline could be that the OC tries to find Natsu after he leaves Fairy Tail. The OC might have a special bond with Natsu and be determined to bring him back. They could go on a journey where they face various challenges and meet new characters along the way. Another idea is that the OC is actually the reason Natsu leaves Fairy Tail in the first place. Maybe there was a misunderstanding or a conflict between them that led to his departure, and the story could be about how they resolve it.
One reason could be that the fanfiction author is trying to create a new and edgy storyline by making Natsu a more negative character. But this often doesn't sit well with fans who love the original positive and passionate Natsu.
Maybe Natsu gets injured during a really tough battle and as a result, he forgets everything about Fairy Tail. The story could then revolve around how the guild members try to jog his memory. They might take him to all the important places in the guild, introduce him to his closest friends again, and even try to recreate some of the battles he once fought in, hoping that something will trigger his memory to come back.