It could be a long - lost relative of an old enemy. For example, if there was a powerful dark wizard that Natsu defeated in the past, this relative might come seeking revenge and be powerful enough to kill Natsu.
Maybe it's a character who has been corrupted by dark magic. This character could start off as a somewhat neutral or even friendly character but then becomes consumed by power and sees Natsu as a threat to their new - found power and kills him.
There could be an ancient evil that has been awakened. This entity might be so powerful that it doesn't even view Natsu as a worthy opponent at first, but then in a sudden and brutal encounter, it ends Natsu's life.
It could be a power - hungry Pokemon trainer. This trainer might see Gladion as a threat to their goal of dominating the Pokemon league or the entire Pokemon world. So, they plan and execute his killing.
Sasuke could turn into the main antagonist if Naruto is killed. Their bond was so strong, and Naruto's death might break Sasuke completely, leading him to go against everything Naruto stood for and become a new threat.
It could be a dark wizard who wants to use Natsu's connection to the Dragonoid for his own evil purposes. This wizard might have set the trap to either steal Natsu's power or to use him as a sacrifice to gain more power related to the Dragonoid.
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.
It could be another dark mage who sees the potential in baby Natsu and wants to use him for evil purposes. This mage might view Zeref as an obstacle in getting to Natsu.
Maybe there's a dark wizard who has a vendetta against Natsu's family. When he finds out that Natsu is secretly a girl, he sees it as an opportunity to further his evil plans. He could try to kidnap her or use her as a bargaining chip against her family or friends. He might think that because she's a girl, she'll be easier to manipulate or capture compared to if she was thought to be male.
It could be a dark wizard who wants to use the baby's power for his own evil deeds. Maybe the baby has some special magic that the wizard thinks can help him take over the magical world.
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.