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.
The World Government could be the main antagonist. They see both Luffy as a growing threat to their rule as a pirate and Dragon as a revolutionary. When Luffy and Dragon meet, the World Government might send their strongest agents like the Admirals to stop them from joining forces. The World Government's strict control over the world and their desire to maintain the status quo would put them in direct opposition to Luffy and Dragon.
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.
Maybe Akainu from One Piece (in a crossover sense within the fanfiction). He could be warped into this Naruto - Dragon Slayer world and his extreme sense of justice could be twisted into a form of tyranny. He sees the chaos and magic in this new world as a threat that needs to be eliminated, and in his overzealousness, he becomes the main antagonist, causing destruction and pitting the ninjas and dragon slayers against him.
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.
It could be an evil water mage who was once a student of the same master as Juvia but turned to the dark side. This antagonist would be jealous of Juvia's natural talent and would try to undermine her at every turn. He might use his knowledge of water magic to create dangerous illusions or floods to harm Juvia and those she cares about.
The dragon god itself could be the main antagonist. Given its power and the threat it poses to the Naruto universe, it would be a natural choice. It could be a being that wants to destroy the ninja world or enslave it for its own purposes.
It could be a rogue ninja who has also somehow gained crystal dragon slayer powers but uses them for evil. He might be jealous of Naruto's natural talent and popularity, so he tries to destroy Naruto and take over the ninja world using his newfound powers.
There could be a secret organization that believes the golden dragons are the key to a new world order. They oppose Naruto's attempts to slay the dragons because they want to harness their power for their own, often evil, purposes. They might send their most powerful assassins after Naruto to stop him.
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.