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 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.
A young orphan who was raised by a hermit in the mountains and discovers they have the mountain dragon slayer magic. They are initially naive but grow into a powerful slayer through their adventures.
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.
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.
Natsu could be one of her potential allies. Since he is a dragon slayer himself, he could understand some of the challenges Lucy faces as a Dark Dragon Slayer. He might offer his fire - based powers to complement Lucy's dark dragon slaying abilities. Another could be Erza. With her strong combat skills and leadership, she would be a great asset in battles against dark dragons. Erza could also protect Lucy when she is still learning to control her new powers.
It could be an original character created specifically for this fanfiction. Maybe a young wizard or witch who has always been an outcast in the wizarding world and discovers their unique ability to connect with dark dragons.
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.
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.