Another plot could be that Natsu's transformation is not complete at first. He has to go on a journey to fully become a dragon. Along the way, he meets different characters who either help or hinder his transformation. Some might be afraid of him, while others see the potential in his new form and want to use it for their own purposes.
Perhaps in the fanfiction, when Natsu turns into a dragon, he can communicate with other dragons in a way he couldn't before. This could lead to him uncovering secrets about his past and the origin of dragons in that fictional world. He might find out he has a special destiny related to the dragons.
One idea could be that Hiccup turns into a dragon during a battle and uses his new form to surprise the enemy. He could fly faster and breathe a more powerful fire than any other dragon, leading his team to victory.
Well, a cool plot might involve Natsu being transported to a parallel world where he is the only water dragon slayer. He has to adapt quickly to this new role and use his skills to fight against an evil force that is draining the water from that world. Also, he could meet alternate versions of his friends and have to convince them to trust him, since his powers are so different from what they know.
Another plot could involve Natsu turning into a dragon in a completely different world. He has to adapt to this new world as a dragon and find a way to turn back. Along the way, he meets other dragons who teach him about his new form and its capabilities. This could lead to him having a greater understanding of his own powers when he finally returns to his normal self.
In this 'natsu turns into a dragon fanfic', a great plotline could be that Natsu's dragon form is different from the typical dragons. His friends, like Lucy, Gray, and Erza, try to find a way to reverse the transformation at first. But as they see the potential in his new form to fight a greater evil, they change their minds and instead help him master his dragon powers. This could involve a series of adventures where Natsu has to learn to fly, breathe different types of dragon fire, and communicate with other dragon - like creatures.
One idea could be exploring Natsu's struggle to control his half - dragon powers. Maybe he accidentally causes destruction in the town and has to deal with the guilt and the fear of others. Another could be a story where Natsu discovers a hidden dragon clan related to his half - dragon heritage and has to prove himself to them.
A great plot could be about Natsu and the ice dragon slayer (let's say a new character) teaming up to defeat a powerful dark wizard. They have to overcome their differences in magic types and personalities at first. Maybe the ice dragon slayer has a cold and reserved nature compared to Natsu's hot - headedness. As they fight together, they learn from each other and create new combined attacks. For example, Natsu could use his fire to heat up the air around the ice dragon slayer's ice attacks, making them more powerful and unpredictable.
Perhaps a plot where Natsu and Wendy are transported to a parallel world where their roles as dragon slayers are completely different. In this world, they are the only hope against an evil dragon that has enslaved all the other dragon slayers. They must quickly adapt to this new reality and use their sibling bond to figure out how to defeat the dragon.
In some fanfictions, Natsu might have different dragon slayer magic elements. Instead of just fire, he could have a combination of fire and lightning, which would make his attacks even more powerful and unique. Also, there could be stories where Natsu travels to different dimensions or time periods, which would introduce all kinds of new challenges and adventures for him. This could lead to him meeting alternate versions of his friends or even new characters.
One idea could be that in a post - Dragon Cry adventure, Natsu and Lucy are sent on a mission to a hidden island where ancient dragon magic is said to be sealed. Along the way, they face various obstacles that test their bond, like a powerful sorcerer who tries to split them up. Another plot could involve Natsu losing his dragon slayer powers temporarily, and Lucy having to protect him while they search for a way to restore his powers using the knowledge from the Dragon Cry incident.
It could be that in this fanfiction, Natsu's dragon powers are enhanced in unique ways. For example, he might gain the ability to communicate with all types of dragons, not just Igneel. This could lead to him having to mediate between different dragon factions. And he may also have to hide his newfound nature from his friends at Fairy Tail for a while, creating some interesting internal conflict.