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.
He could also face challenges within his own guild. Some of the members might tease him, while others might be overly protective. Natsu as a girl would have to navigate through these new social dynamics. Also, his appearance as a girl might attract unwanted attention from some villains who see him as an easier target, but he would prove them wrong with his strength.
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.
One possibility could be a great tragedy that befalls him. For example, if all his friends were brutally killed in front of him by an enemy he couldn't defeat at that time. This might shatter his belief in the goodness of the world and drive him to become cold - blooded in order to seek revenge. He would start to see killing as the only way to get justice.
Well, perhaps Natsu could be turned into a vampire during a battle with a powerful dark magic user. Then he'd struggle with his newfound vampiric instincts, like the urge to drink blood, while still trying to be true to his friends and his guild.
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.
Well, in this kind of fanfiction, Natsu might teach his apprentice about magic. He could pass on his unique fire - based magic skills, like how to control the intensity of the flames. And they might go on adventures together, facing various challenges and enemies. Maybe the apprentice has some special abilities that Natsu has to help develop.
In a 'Goku meets Natsu' fanfiction, they might start off by being confused about each other's powers. Goku could be intrigued by Natsu's fire - based magic, while Natsu would be amazed at Goku's super Saiyan strength. They could end up having a friendly sparring match to test each other's skills.
In a 'naruto meets natsu fanfiction', they might start off as rivals. Naruto, with his ninja skills and determination, and Natsu, with his dragon - slayer strength. But as they get to know each other better, they realize they have a lot in common. For example, they both have a strong sense of loyalty to their friends. So they end up becoming good friends and combining their powers to protect their respective worlds from threats that could endanger both.
Well, it could be about Natsu suddenly finding himself in a situation where he has multiple dragon companions who are all attracted to him. Maybe he has to deal with their different personalities and powers while also trying to figure out how he got into this harem situation in the first place.
One possible plot could be that Natsu is cursed by a dark wizard. This curse slowly transforms him into a fire dragon. As he changes, he struggles to control his new dragon - like instincts and powers. His friends initially fear him but then realize they need to help him break the curse.