Perhaps the main antagonist is an ancient dragon that has awakened from a long slumber. This dragon views Natsu as an abomination because he is a human turning into a dragon, and it decides to hunt him down to put an end to what it sees as an unnatural process.
Another option is a rival dragon slayer who sees Natsu's transformation as a threat to his own status. This rival could try to stop Natsu from fully becoming a dragon or try to defeat him to prove his own superiority.
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.
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.
It could be a dark wizard who wants to capture Hedwig - as - a - dragon for her powerful new form. Maybe he wants to use her dragon abilities for his own evil plans.
The main antagonist could be an evil knight who has a vendetta against dragons. When the princess turns into a dragon, he sees her as a prime target and tries to slay her, believing all dragons are evil. He doesn't care that the princess was once human and is only focused on his own prejudiced views.
There might be a group of anti - magic fanatics as the main antagonists. They view Natsu's new and extremely powerful magic as a danger to the natural order of the magical world. They would try to eliminate Natsu and any other powerful mages in Fairy Tail who support him, believing that magic should be limited and controlled, not allowed to reach such overpowered levels as Natsu has.
It could be a rogue Dragon Slayer who is jealous of Lucy's new - found powers. This character might have been exiled from their own guild for misusing their powers and now sees Lucy as an easy target to prove their superiority.
Well, perhaps the story could start with Natsu having a strange encounter that begins his transformation into a dragon. Maybe he discovers some ancient magic or a hidden power within himself that is related to dragons.
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 a dark wizard who wants to control the dragons for his own evil purposes. He might see Harry as a threat because Harry is forming a bond with the dragons and teaching them to resist dark magic.