It could be an evil wizard. There are many powerful and malevolent wizards in the Fairy Tail universe who might target Natsu for some reason and use dark magic to make him cold.
Perhaps it's an old enemy from Natsu's past that has resurfaced. This enemy might have a vendetta against Natsu and decides to change his personality as a form of revenge. It could be someone related to the dragon that taught Natsu magic, and they blame Natsu for something that happened to the dragon.
It's possible that Natsu's coldness is a temporary state. He could be under a spell that makes him seem cold, but deep down his true self is still there. As the story progresses, his friends might try to break the spell and bring back the old Natsu. They could search for a cure or a way to reverse the magic, leading to many adventures and challenges along the way.
Maybe it's a fellow guild member who has made a series of bad decisions. Their actions keep putting the guild in danger, and Natsu has been warning them. But when the situation gets really bad and people get hurt, Natsu snaps at that guild member for their irresponsible behavior.
Lucy could also play a major role. She might be the one to help Natsu adjust to his new life as a girl. Maybe she gives Natsu tips on how to dress and act like a girl, and they could have deeper conversations about what it means to be a girl in their magical world.
Maybe an ancient dragon that has been awakened from its slumber and views Natsu as an intruder in the dragon world. Since Natsu is a new - made dragon, this old dragon might try to drive him out or even harm him.
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.
Maybe Natsu discovers a new magic spell that accidentally turns him into a dragon. He then has to figure out how to reverse it while dealing with the new powers and instincts.
Perhaps an evil mage who has been researching on how to control dragons. When he hears about Natsu's transformation, he sees Natsu as a perfect specimen to experiment on. So he becomes the main antagonist, trying to capture Natsu and use his powers for his own dark purposes.
It could be an evil dragon slayer who is jealous of Natsu's new form. This antagonist might try to steal Natsu's power or find a way to turn him back to his normal self out of envy.
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.