The main antagonist could be a powerful and ancient fairy who has been banished long ago but now returns to seek revenge on the fairy world. And Lucy, being a new and important part of the fairy world, becomes a target. This antagonist might have unique and dark magic that makes it extremely difficult for Lucy to defeat. Also, the antagonist could have some sort of grudge against the fairy queen or king, and Lucy is caught in the middle of this power struggle.
It could be an evil sea witch. She might be jealous of Lucy's newfound mermaid powers or her connection to the Fairy Tail guild. The sea witch could try to steal Lucy's powers or use her to gain access to the land world.
It could be a new character introduced specifically for the fanfiction. Maybe a powerful mage who has a vendetta against Lucy and somehow manages to drive her out of Fairy Tail by causing trouble for her within the guild.
A powerful magical being from another dimension could be the main antagonist. This being senses the departure of Lucy and Juvia from Fairy Tail and sees it as an opportunity to invade their world. It tries to stop the two girls from getting help or finding a way to stop it, as they are two of the most powerful mages outside of the guild protection now.
A possible main antagonist could be Aizen from 'Bleach' who somehow gains the ability to control the powers from Lucy's world as well. He could use this newfound power to try and take over both the Bleach universe and Lucy's world. His cunning and powerful abilities would make him a formidable foe for Lucy and the Bleach characters to face.
A time - traveling bounty hunter could be the main antagonist. He might be after Lucy because she has somehow disrupted the time - space continuum. He's cold - hearted and will stop at nothing to bring her to some time - traveling justice.
It could be a dark wizard who wants to use the baby's power for his own evil deeds. Maybe the baby has some special magic that the wizard thinks can help him take over the magical world.
It could be another powerful celestial dragon who opposes Lucy's ideas or presence.
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.
A powerful sorcerer who wants to control the power of the fairy of darkness for his own evil purposes could be a great antagonist. He might try to trap her and force her to do his bidding.
It could be an evil fairy king. He might see the poison fairy as a threat to his rule. Maybe he believes that her powers are too dangerous and wants to either control her or get rid of her. He could send his minions after her and try to spread false rumors about her in the fairy community.