It could be another powerful celestial dragon who opposes Lucy's ideas or presence.
Maybe a group of humans who have a long - standing feud with the celestial dragons and see Lucy as a traitor for being on their side.
Perhaps an ancient sorcerer who has the ability to control celestial dragons. He sees Lucy as an obstacle to his domination over the world using the power of these dragons. He would use his vast knowledge of magic and his control over the dragons to try and defeat her.
A powerful and evil dragon could be the main antagonist. This dragon might have a grudge against the dragon slayers in general and specifically targets Lucy because of her growing reputation as a star dragon slayer.
It could be an evil ice dragon that has gone mad with power. This dragon might be hoarding all the ice magic in the land, causing problems for the people and other magical creatures. Lucy would then have to face this powerful foe to restore balance.
A powerful dark mage who wants to use Lucy's newfound dragon slayer powers for his own evil purposes. He might try to control her or steal her powers.
It could be a dark wizard who wants to use Lucy's new celestial spirit powers for his own evil purposes. He might try to capture her and force her to do his bidding.
Well, obviously Lucy is one of the main characters since it's in the title. She's likely to be the central figure around which the story revolves. There might also be other dragon slayers who could be important characters. They could be friends or rivals to Lucy, and their interactions with her would shape the story.
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 about Lucy having an adventure in the world where celestial dragons exist. Maybe she befriends one or tries to change their cruel ways.
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.