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.
There could be a group of anti - dragon fanatics. They view Natsu's transformation into the fire dragon as an abomination and will stop at nothing to get rid of him. They might use advanced technology or ancient anti - dragon magic to try and take him down. Their leader could be a former dragon slayer who had a bad experience with dragons and now wants to wipe out any form of dragon - like existence, including Natsu in his new form.