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.
One possible plot could be that Natsu is cursed by a dark wizard. This curse slowly transforms him into a fire dragon. As he changes, he struggles to control his new dragon - like instincts and powers. His friends initially fear him but then realize they need to help him break the curse.
Well, perhaps the plot could involve Natsu's transformation being a result of some powerful magic or a dark curse. He might struggle to control his new dragon form at first, causing chaos among his friends and allies.
Perhaps an evil mage who has been researching on how to control dragons. When he hears about Natsu's transformation, he sees Natsu as a perfect specimen to experiment on. So he becomes the main antagonist, trying to capture Natsu and use his powers for his own dark purposes.
It could be an evil dragon slayer who is jealous of Natsu's new form. This antagonist might try to steal Natsu's power or find a way to turn him back to his normal self out of envy.
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.
Obviously, Natsu is a very popular character. He is the main focus as the fire dragon slayer. His hot - tempered and determined nature makes him an exciting character to write about.
A powerful dark dragon slayer who has mastered both shadow and ice magic could be a great antagonist. He could be jealous of Natsu's natural ability with fire and his popularity among the guild, and thus wants to prove his superiority.