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.
Maybe a dark wizard who wants to use the power of ice dragons for his own evil purposes. He sees Lucy as a threat to his plans because she is an ice dragon slayer. So he constantly tries to stop her, sending his minions after her and setting up traps. He could also be trying to create an army of ice - controlled creatures using dragon magic, which Lucy has to prevent.
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.
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.
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 dark dragon slayer who has mastered both shadow and ice magic could be a great antagonist. He could be jealous of Natsu's natural ability with fire and his popularity among the guild, and thus wants to prove his superiority.
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 dragon - themed pirate could be the main antagonist. This pirate has a crew of dragon - like creatures and they are causing chaos across the seas. They have a grudge against Luffy for some reason, perhaps because Luffy has unknowingly entered their territory or stolen something valuable from them.
A powerful dark dragon that was once sealed away but has now broken free. It could have a vendetta against all dragon slayers, especially Gray.
It could be another powerful celestial dragon who opposes Lucy's ideas or presence.
A young orphan who has been training to be an ice dragon slayer since a young age. They have a strong will to prove themselves in a world that has always looked down on them because of their lack of family and resources.
It could be an evil ice mage who wants to use the Ice God Slayer's power to freeze the entire world. This mage might have a tragic past that led them to believe that freezing the world is the only way to create peace.