It could be an evil sea witch. In a fairy - tale - inspired fanfiction, an evil sea witch with the power to control tentacle - like sea creatures would be a great antagonist. She might see Ezra as a threat or simply want to use Ezra for her own dark purposes.
There's a chance that the main antagonist is a wizard gone bad. He might have created these tentacle - like apparitions as part of an experiment or a spell gone wrong. Now, these tentacles are causing trouble for Ezra, and the wizard either doesn't care or is actively trying to stop Ezra from interfering with his plans.
Maybe in the 'ezra struggles in tentacles fairy tale fanfiction', Ezra is a character who has been lured into an underwater world. In this world, there are tentacle - like creatures that are trying to keep Ezra there for some unknown reason. Ezra has to use their wits and any magical abilities they might have to escape from these tentacle - based imprisonments. It could be a journey of self - discovery as well as a fight for freedom.
Obviously, Kidomaru would be a main antagonist as he is the one with the armsgrip on Sakura, causing her to struggle.
Perhaps a powerful dark side cult leader. They could manipulate Ezra into believing that their way is the right one and that the dark side is a misunderstood power. The cult leader might use Ezra's own insecurities and desires to draw him into the dark side.
It could be a dark wizard. Dark wizards are often antagonists in fanfictions. He might have created the black tentacles as a way to capture Kairi for some dark purpose, like using her powers for his own evil deeds.
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.
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.
Another idea is that the main antagonist is actually an aspect of Starfire herself, represented by the 'Oozing Red X'. It could be some sort of dark side of her that has manifested in this form. So she's not just fighting an external foe, but also this internal struggle that is personified by the 'Oozing Red X'.
It could be a former Fairy Tail member who turned to the gangster life. They know all the guild's secrets and weaknesses, so they can be a very formidable opponent. They might have left the guild because they felt betrayed or didn't get the recognition they thought they deserved.
It could be an ancient dragon that has been awakened from a long slumber. This dragon might have a vendetta against Ignia or the Fairy Tail guild for some past transgression.