A possible plotline is that Natsu's dragon form attracts the attention of dark wizards who want to use his power for evil. So he has to fight against them while also coming to terms with what it means to be a dragon. He may also have to deal with the physical and mental changes that come with this transformation, like different instincts and a new way of seeing the world.
One interesting plotline could be Lucy and Natsu going on a quest to find a lost dragon egg. Along the way, they face many challenges and their bond grows stronger. Another could be Natsu suddenly losing his dragon powers and Lucy having to find a way to get them back, which involves delving into some ancient magic lore.
Another plotline might involve the dragon slayers teaming up against an ancient evil that threatens the entire world of magic. They would have to combine their unique dragon - slaying abilities to defeat it, like Natsu's fire - based attacks with other slayers' powers.
One interesting plotline could be Natsu facing off against a powerful new dragon that has abilities he's never encountered before. This forces him to combine his skills as a multi - dragon slayer in unique ways.
One possible plotline could be Natsu developing unique magical abilities different from his canon ones due to being raised by another dragon. Maybe he has a different way of controlling fire or even some new elemental powers.
One interesting plotline could be Natsu discovering new dragon powers that are unique to him. This could lead to him having to learn how to control these powers and facing challenges within the Fairy Tail guild as others may be wary or envious of his new abilities.
One interesting plotline could be Natsu going on a quest to find a long - lost dragon that was rumored to have special powers. This could involve him facing numerous challenges and making new friends along the way.
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.
Well, perhaps it could be about Natsu having some special powers right from birth that are related to dragons. Maybe he can communicate with dragons in a unique way that no one else can since he was born with this dragon - like essence.
One common plotline could be how they handle their daily lives together while still being part of their guild. Maybe they have to balance their relationship with their duties as mages.
One interesting plotline could be a time - travel story. For example, natsu, lisanna, and mirajane could be sent back in time to when they were younger and have to deal with past events while also trying not to change the future too much. It would be full of nostalgic moments and also opportunities for character growth.