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.
It could also be one of his close friends. Maybe Natsu's actions not only put himself in danger but also endangered his friend. In that case, out of concern and frustration, his friend would berate him for his irresponsible behavior.
Well, it could be that Natsu made a big mistake during a mission, like accidentally destroying a precious artifact that was crucial for the guild's future. So the guild master berates him for his carelessness.
In some fanfictions, it could be Gray. Their rivalry - turned - friendship has a lot of potential for such an unexpected plot twist where Gray is the father.
It could be an enemy dragon. There are many rival dragons in the Fairy Tail universe, and if they see Natsu with Igneel, they might target Natsu to get to Igneel. Dragons can be very territorial and protective of their own, and an enemy dragon may see Natsu as a weak point to attack Igneel through.
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.
It could be a jealous rival from another guild. They might see Natsu as a threat and figure framing him would be a good way to get him out of the way.
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.
He might meet a powerful wizard who is the ruler of that dimension. This wizard could initially be hostile towards Natsu, seeing him as an intruder, but later they could become allies.
Natsu's friends like Gray, Erza, and Lucy are often involved. Gray might be constantly teasing Natsu about the transformation, creating some funny banter between them. Erza could be the one who tries to keep order and protect Natsu in this new form. Lucy might offer more emotional support, sharing girl - related experiences with Natsu to help her adjust.
I'm not sure who exactly Natsu is just from 'natsu reads fanfiction'. It could be a normal person who just enjoys reading fan - made stories. Maybe it's a nickname for someone within a particular fan community.
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.