There might also be some dark magic - using villains who want to use Natsu's alpha dragon slayer powers for their own evil purposes. They could try to capture him or manipulate him in some way. Maybe they plan to extract his power to create a powerful weapon or something.
It could be a powerful dark wizard who sees Natsu as a threat because of his alpha status among dragon slayers. This wizard might want to eliminate Natsu to gain control over the dragon slayers or to carry out some evil plan without interference.
One possible interesting plot point could be Natsu's unique powers as the alpha dragon slayer. Maybe he has enhanced strength or special abilities that set him apart from other dragon slayers. For example, he could have a more powerful form of dragon roar.
Some fanfictions feature ancient evils that were sealed away long ago but are now starting to break free. These could be powerful demons or corrupted spirits. They see Lucy and Natsu as threats to their plans of world domination, so they target them. The antagonists in these cases are not only physically powerful but also very cunning, constantly devising new ways to outwit our heroes.
In some fanfictions, there might be original characters created by the author. For instance, a mysterious figure who has some connection to Natsu's multi - dragon slayer abilities. This character could be an antagonist, testing Natsu's skills, or a mentor, guiding him to master his powers better.
Obviously, Natsu is one of the main characters. He is the strongest dragon slayer in this fanfiction. His friends from Fairy Tail like Lucy, Gray, and Erza are also likely to be main characters. They often support Natsu in his battles and adventures.
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.
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.
One of the main characters could be Happy, Natsu's loyal flying cat companion. Happy would likely be by Natsu's side during his retirement, still providing his comical relief and support. Another character could be Lucy. She might be concerned about Natsu's retirement decision and try to understand his reasons, or she could also be retiring from her own magical adventures and join Natsu in his new peaceful life.
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.
One common theme is power and growth. As Natsu becomes a water dragon slayer, he has to learn to control his new powers, and the fanfictions often explore how he grows stronger. For example, he might face challenges that force him to adapt quickly.