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 Morgana. She might see Merlin's transformation into a dragon as a threat to her own power and plans. So, she tries to stop him from fully becoming a dragon or tries to use his new form for her own evil purposes.
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.
An ancient dragon that has turned evil due to some dark magic could be the main antagonist. This dragon might see Natsu as a threat because of his unique combination of powers and wants to eliminate him. Natsu would then have to use all his skills and the help of his friends to fight this dragon. The dragon could also have minions that are constantly causing trouble for Natsu and his guild.
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 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.
Maybe it's another dragon, a malevolent one, who sees the female dragon's act of raising a human as an abomination. He decides to target Natsu to prove a point or to gain power over the female dragon. The malevolent dragon could try to undermine Natsu at every turn, creating numerous challenges for him.