It could be an existing character like Naruto who, through some strange turn of events, gains the Wood Dragon Slayer power. This would completely change his fighting style and how he approaches challenges. He could use the power in creative ways, like combining it with his Rasengan to create a new, more powerful attack.
It could be a new character created specifically for this fanfiction. This character could be a young water dragon slayer who gets lost between the two worlds. They would be the main focus as they try to adapt to the Naruto world while still having their Fairy Tail powers.
Perhaps a young and inexperienced adventurer who discovers their hidden power while on the quest to slay the dark dragon. They grow throughout the story, learning new skills and becoming a true hero.
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 a powerful dragon who has been corrupted by dark magic. This dragon might see Naruto's newfound powers as a threat and decides to target him and the Hidden Leaf Village first.
It could be an evil shinobi who has somehow harnessed shadow dragon slayer magic for his own evil purposes. He uses it to create chaos and destruction in the ninja world, and Naruto and the dragon slayer have to stop him.
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.
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 organization of rogue ninja who have also somehow gained similar powers but use them for destruction. They see Naruto as a threat to their plans of taking over the ninja world with their newfound powers. They constantly try to undermine Naruto and steal his unique Crystal Dragon Slayer techniques.
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.