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.
A love interest who breaks his heart multiple times. For example, if this person plays with his feelings, lies to him, or cheats on him, Percy could become cold - hearted as a form of self - protection. He would no longer trust easily and would close his heart to new relationships.
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.
There could be a female dragon named Ignis who is very fiery and passionate, always trying to be the closest to Natsu. Another could be a more gentle dragon named Aqua, who has water - based powers and uses her calm nature to balance out the more intense dragons in the harem.
In addition to Lucy and Erza, there could be other female characters from the series. Maybe some minor characters who have always admired Natsu from afar. For example, that one girl from the guild who always blushes when Natsu is around. Another possibility is a powerful female dragon who sees Natsu's transformation into a Dragon God as a sign of his worthiness. She might join his harem, bringing a different kind of relationship dynamic as she has her own dragon - like qualities and power. And there could be a female mage from a different guild who hears about Natsu's new status and is intrigued, eventually becoming part of his harem.
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.
It could be an old and powerful vampire lord who sees Natsu as a threat to his rule. Since Natsu is a new and different kind of vampire, the lord might want to eliminate him.
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.