Well, it could be a really dramatic turn. Natsu might initially be in a state of shock. His friends would surely be horrified and rush to his aid. Maybe his dragon - like powers would start to flicker as he tries to fight against the injury.
Natsu's friends would have a range of reactions. Lucy would be distraught, her face pale with fear as she kneels beside Natsu. Gray would clench his fists, his eyes filled with rage as he scans the area for the shooter. Erza would be the voice of calm and reason, though her hands would be shaking slightly as she tries to stop the bleeding. Happy would be flying around in a frenzy, screeching in distress. They would all be united in their determination to save Natsu and seek justice for what has happened.
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.
Perhaps a mysterious stranger who has some unknown vendetta against the Heartland family. This person could have been lurking around the ranch for a while, waiting for the perfect opportunity to strike. Or it could be someone within the community who has a hidden agenda and sees Amy as an obstacle to their plans.
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.