One possibility is that Natsu discovers some ancient dragon magic that transforms him into a Dragon King. He might gain new powers like the ability to control all types of dragons or access to long - lost dragon knowledge. This could lead to him having to face new enemies who are threatened by his new - found status.
His relationship with his guildmates would likely become more complex. They might look at him with a mix of awe and a bit of fear. His closest friends would still support him, but they would have to get used to his new - found power.
Maybe it could involve Natsu having to go through some really tough trials to prove himself worthy of becoming the Fire Dragon King. For example, he might have to battle powerful enemies that are related to the dragon world's ancient secrets.
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.
Maybe Natsu discovers a new magic spell that accidentally turns him into a dragon. He then has to figure out how to reverse it while dealing with the new powers and instincts.
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.
Well, it depends on the specific fanfiction. There are various interpretations in the fanfiction world. Some might make Natsu the Fire Dragon King to explore different storylines, like him having greater power and more responsibilities. This can lead to really interesting character development and new plot twists. For example, he could face different challenges and enemies compared to his normal state in the original story.
His relationship with Lucy would become more complex. She might be in awe of his new power but also worried about losing the Natsu she knew. His friendships in the guild could also be strained as they might see him as a different person now. Some might be jealous of his new status, while others would be afraid that he'll leave them behind.
There could be a female dragon named Ignis who is very fiery and passionate, always trying to be the closest to Natsu. Another could be a more gentle dragon named Aqua, who has water - based powers and uses her calm nature to balance out the more intense dragons in the harem.
In addition to Lucy and Erza, there could be other female characters from the series. Maybe some minor characters who have always admired Natsu from afar. For example, that one girl from the guild who always blushes when Natsu is around. Another possibility is a powerful female dragon who sees Natsu's transformation into a Dragon God as a sign of his worthiness. She might join his harem, bringing a different kind of relationship dynamic as she has her own dragon - like qualities and power. And there could be a female mage from a different guild who hears about Natsu's new status and is intrigued, eventually becoming part of his harem.