There could be a powerful demon from the underworld. This demon has heard about Sarah's power of control and believes that if he can take it from her, he can rule not only the fantasy world but also break through to the human world. So he sends his minions to harass Sarah and tries to find her weakness so that he can defeat her and steal her power.
The plot could involve Sarah discovering she has some special power in a magical world. Maybe she can control elements like fire or water. And she has to use these powers to save her land from an evil sorcerer.
It could be an evil sorcerer. This sorcerer might be jealous of Sarah's unique power of control and wants to take it for himself. He could constantly send his minions to attack Sarah and try to disrupt her journey of self - discovery and power - mastering.
Maybe there is a dark force within the Goblin Kingdom itself. Some of the goblins might not be happy with Jareth having a half - human baby as his heir. They could plot against the baby and Sarah, trying to undermine Jareth's rule and get rid of the baby. Sarah and Jareth would then have to deal with this internal threat while also protecting their baby from outside forces.
The ex - husband is most likely to be the antagonist in such a blackmail ex - wife fantasy story. He might be bitter about the divorce and think that blackmailing his ex - wife is a way to get what he wants, whether it's money, property, or revenge.
In a 'Controlling Sarah' fantasy story, perhaps Sarah discovers she has the power to control the elements. She could start by taming wild fires that threaten her village. With her newfound power, she has to learn to control it so as not to cause chaos. Maybe she gets mentored by an old wizard who teaches her the ethics of power control.
It could be a disgruntled client. Maybe the female attorney didn't win a case for this client as they hoped, and in revenge, they dig up some dirt on her to blackmail her.
A powerful sorcerer who is jealous of the boy's magic and wants to stop him from using it on the females. This sorcerer might see the boy's actions as a threat to his own power or plans.
The antagonist could be the scientist who was trying to stop the invisible man from misusing his power. Maybe he created the invisibility in the first place but didn't want it to be used for bad things.
The witch is the obvious antagonist. In a horror context, she could be more than just a jealous old woman. She might be a powerful sorceress with dark motives like sacrificing Rapunzel for some evil ritual. Her control over Rapunzel and the tower makes her a menacing figure.
Perhaps a new character, a dark sorcerer. This sorcerer might be jealous of Sonic's popularity and power. So, he casts a spell on Sonic that makes him act out of character. In this 'dark sonamy story', Amy has to face this sorcerer who is much more powerful than she is, but she's determined to save Sonic no matter what.