Perhaps an ancient dragon that has been sleeping for centuries and wakes up to find Astrid turning into a dragon. This dragon sees Astrid as an abomination because she was not born a dragon but is becoming one through some unnatural means. So, it decides to hunt her down and destroy her.
Perhaps a jealous wizard. He was once friends with Astrid but when he sees her gain the power and beauty of a dragon, he becomes envious. He starts to cast spells to make Astrid's life as a dragon difficult and tries to steal her new - found powers for himself.
One possible plot could be that Astrid is cursed by an evil sorcerer. At first, she has no idea what's happening to her but slowly starts to notice physical changes like her skin getting scaly. As she turns more and more into a dragon, she has to struggle to maintain her relationships with her friends who are initially afraid of her new form. Eventually, she discovers a way to break the curse through an ancient magic ritual.
It could be a dragon hunter who sees Hiccup in his dragon form and wants to capture him for study or to use his unique abilities for evil purposes.
In the fanfic, Astrid could be part of a scientific experiment gone wrong. The experiment was meant to study dragon DNA, but something malfunctioned and she starts to transform. She then embarks on a journey to find a cure. Along the way, she discovers that being a dragon gives her a unique perspective on the world and she has to decide if she really wants to go back to being fully human.
It could be an ancient dark wizard who has been manipulating events from the shadows. This wizard might see Elsword as a powerful tool and has been slowly influencing him to turn dark. The wizard could be using dark magic to cloud Elsword's mind and make him more susceptible to negative emotions.
Bowser could be the main antagonist. He might see Luigi's transformation as an opportunity to mess with Mario and the Mushroom Kingdom. He could try to use Luigi's new form to his advantage, perhaps by trying to make Luigi a spy or by using his new identity to cause chaos among the kingdom's residents.
Perhaps an evil mage who actually intended for the transformation to go wrong on purpose. They might want to use Sans' new female form to cause chaos in the Underground. They could be trying to manipulate Sans into doing bad things or using her to get to other important characters.
It could be a jealous rival. Maybe someone who has always been envious of Robin's abilities or popularity. Now that Robin is in a more vulnerable state as a girl, this rival sees an opportunity to undermine him.
It could be a witch who cast the spell that turned Dean into a girl. This witch might have some ulterior motives, like using Dean's new form to disrupt the balance in the hunting world. Maybe she wants to create chaos among hunters by making them question Dean's abilities now that he's a girl.
A new villain could emerge who is specifically targeting adult heroes. Since Deku has now become an adult, he becomes a prime target. This villain might have a grudge against the hero society's adult leadership and see Deku as a symbol of that.