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 possible scenario could be that Finn, after being turned into a vampire, struggles with his new identity. He might have to deal with the urge to drink blood while still holding onto his old values. For example, he could try to find alternative ways to satisfy his thirst that don't involve harming humans, like drinking animal blood.
One idea could be that Stiles is turned into a vampire against his will. Maybe he's attacked one night while out investigating something supernatural. As a vampire, he has to struggle with his new urges, like the thirst for blood, while still trying to be the same person his friends know. He might start to see the world in a different light, both literally and metaphorically, as his vampire senses heighten.
One possible story could be that Mimiko is bitten by a mysterious vampire one night while walking alone. She then starts to experience strange cravings and enhanced senses. At first, she's terrified, but as time goes on, she begins to embrace her new vampiric nature.
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.
One possible scenario in this fanfiction could be that Finn's transformation into a vampire brings out a whole new side of his personality. He might struggle with his newfound thirst for blood while still trying to uphold his heroic values. Maybe he teams up with Marceline, who is already a vampire, and she teaches him how to control his urges.
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.
Well, perhaps the plot could involve Natsu's transformation being a result of some powerful magic or a dark curse. He might struggle to control his new dragon form at first, causing chaos among his friends and allies.
No. Fanfiction that involves non - consensual or inappropriate relationships like this is not acceptable. It goes against basic ethical principles of respect and equality. Such content objectifies characters and can be disturbing and offensive to many readers.
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.