Lucy could be a main character. She was always close to Natsu in the original Fairy Tail. So when Natsu returns as an 'ex' and meets the guild again, she would likely play a big role in how he is received and in the development of the story.
It could be about Natsu having left Fairy Tail for some reason and then reuniting with the guild in a very unexpected way. Maybe he has new powers or a different outlook on life that causes some friction and also some amazing moments of reunion and growth within the guild.
Seilah is also a main character in this context. She has her own set of powers and a rather complex personality in the Fairy Tail universe, which can be further explored in fanfiction.
In fanfiction, Natsu's age can vary greatly. Some authors might keep him at his canonical age from the original Fairy Tail series, which is around 17 - 18 years old at the start. However, in alternate universe fanfictions, he could be older or younger depending on the plot. For example, in a time - travel based fanfic, he could be much older if the story takes place far into the future or much younger if it goes back to his childhood days.
Yes, in a lot of Fairy Tail fanfiction, Natsu can be overpowered. Writers often like to amp up his powers for more exciting storylines. For example, they might give him new fire - based abilities or make his existing ones even stronger to defeat more powerful foes.
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 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.
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.
Gray would be shocked at first. He and Natsu have a long - standing rivalry - but also a deep friendship. He might initially be angry that Natsu would just leave like that, but then he would realize how much he'll miss him. He might also feel a sense of loss for the guild as a whole, since Natsu is such an important part of it.
Obviously, Natsu would be one of the main characters. He's the 'badass' in the title. His fiery personality and powerful magic are likely to be central to the story.
Natsu is the central figure in this fanfiction. But other characters like Happy, his loyal flying cat companion, will surely play an important role. Makarov, the guild master, would also be a main character as he has to deal with Natsu's return, whether it's a welcome or a bit of a shock to the guild's current state. And don't forget about the villains or antagonists that Natsu and Fairy Tail might face upon his return, they could also be considered main characters in the sense that they drive the plot forward.