There could be new human characters as well. Maybe some mages from other guilds who are curious about Natsu's new status. They could either be allies or potential threats. And don't forget about the celestial spirits. Since Lucy is Natsu's friend, the celestial spirits might get involved in some way. They could offer their unique powers or knowledge to help Natsu in his new role as the dragon king.
A young dragon slayer - in - training who idolizes Natsu. This character could be from a far - off land and has heard tales of Natsu's dual powers. They could have their own unique magic related to a different element that complements Natsu's powers.
There could be a young mage who idolizes Natsu and joins him on his adventures. This character could be inexperienced but full of enthusiasm, providing some comical moments as well as opportunities for Natsu to be a mentor.
Another potential character could be a dragon oracle. This oracle could have the ability to see Natsu's true nature and destiny. They could either help Natsu understand his powers and purpose or be a bit of a trickster, making Natsu's journey to discover himself even more difficult. And of course, the other members of Fairy Tail would be there to support or sometimes misunderstand Natsu as he deals with his dragon - born identity.
Potentially, we could see some celestial dragon characters. They might be ancient and powerful beings. Some could be friendly towards Natsu, seeing him as a new hope for their kind. Others might be more hostile, seeing him as an intruder or a threat to their traditional ways. There could also be other mages from different guilds who are either curious or envious of Natsu's situation with the celestial dragons.
In addition to Natsu's Fairy Tail friends, there could be some dragon characters. Maybe Igneel, Natsu's adoptive father dragon, could play a role, perhaps in the form of a spirit or a memory guiding Natsu. There might also be other powerful dragons who are either allies or enemies depending on how they view Natsu's transformation into the Fire Dragon King.
Igneel could also play a role. Since Igneel was Natsu's adoptive dragon father, he could appear in Natsu's mind during the transformation, guiding him and giving him advice on how to handle his new dragon powers. And there could be other dragons in the story as well. They might be either friendly or hostile towards Natsu depending on their nature.
There could be some of Dragon's comrades in the revolutionary army who become important figures in Luffy's upbringing. For example, a veteran revolutionary who teaches Luffy about the history of the world and the wrongdoings of the World Government.
In the 'king of dragon slayer fanfiction', Natsu is likely a central figure. He might have special powers related to slaying dragons, like breathing fire or having enhanced strength. His character could be developed in ways that are different from the original series, perhaps with new relationships and challenges within the fanfiction world.
There could be a mysterious old wizard who takes Natsu under his wing when he leaves Fairy Tail. This wizard might have knowledge of ancient magics that Natsu is eager to learn.
Obviously, Natsu himself would be the central character. His best friend Happy could also be involved, maybe Happy starts to notice some of Natsu's strange dragon - related behaviors first. Gray might also be in the story, perhaps he gets into a fight with Natsu where Natsu's dragon powers accidentally show a bit more than he intended.