Well, first of all, Natsu would probably be excited to see himself in all kinds of wild adventures in the fanfiction. He loves action and if the fanfic has him in some epic battles, he'd be all for it. Lucy might be a bit more critical. She'd look at how she's portrayed in terms of her intelligence and her relationships with the other guild members. Gray would likely be cool about it, but might get a bit annoyed if the fanfic made him seem too cold or too goofy. And Erza would be very strict in her assessment. If the fanfic doesn't adhere to the values of the guild she holds so dear, she might not be too happy about it.
In this fanfiction, perhaps Natsu's return as a 'king' means he has a new level of leadership. He might bring back some lost magic or knowledge that helps Fairy Tail regain its former glory. For example, he could have found a way to defeat an old enemy that was once too powerful for the guild. His return could also stir up emotions among the guild members, like excitement, relief, or even some jealousy from those who might have risen in his absence.
Well, one interesting idea could be the return of the dragons bringing new powers to the Fairy Tail wizards. Maybe they bestow unique magical abilities that help the guild face even greater threats. Another idea might be the re - establishment of old bonds between the dragon slayers and their dragon parents or mentors, leading to some really heart - warming or intense emotional moments.
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.