Maybe Gray has to team up with a dragon in order to defeat an ancient evil that threatens to destroy the world. The dragon could have its own motives and Gray has to figure out if he can trust it.
One interesting plot idea could be Gray facing a new type of Devil that has powers over ice in a different way than he does. For example, it could freeze emotions rather than just physical objects. Another could be Gray having to team up with an unlikely ally from his past to defeat a powerful Devil Slayer - turned - villain. Maybe a Devil has possessed a member of Fairy Tail and Gray has to find a way to exorcise it without harming his friend.
The titanium dragon slayer could be cursed. Every time they attempt to slay a dragon, they are instead inflicted with a part of the dragon's power, slowly turning them into a half - dragon, half - slayer hybrid. This transformation makes them an outcast among their own kind and forces them to reevaluate their role in the world of dragon slaying.
One idea could be that the poison dragon slayer has to overcome a rare and powerful poison - based curse that is slowly sapping their strength. They must find a mythical antidote hidden in an ancient dragon's lair.
Well, he might travel back in time and end up being mistaken for a demon himself because of his unique ice powers. This could lead to all sorts of misunderstandings and battles. Or perhaps he travels back to the time of the first demon slayers and has to train with them using his modern - day knowledge of fighting demons. He could also travel to a time when a major demon - slayer organization was in its infancy and help shape it into a more effective force against demons.
Maybe the Wood Dragon Slayer has a connection to the Sage of Six Paths. This connection gives them unique abilities that no one else has. They could discover this connection during a dangerous mission in an ancient ruin. And then they have to learn how to control these newfound powers while also dealing with the political unrest in the village that arises from others fearing their new strength.
One idea could be Juvia discovers a new water - based magic that only water dragon slayers can use. It gives her the ability to transform water into different forms, like crystal - like structures for defense. Another could be a time - travel plot where Juvia goes back in time and meets the first water dragon slayer and learns ancient techniques from them. And a third idea is that Juvia has to team up with other dragon slayers to defeat a powerful water - based villain who is misusing water magic and causing chaos.
One idea could be Natsu exploring a new magic ability related to his dual dragon slayer powers. For example, he discovers a way to combine the fire of his Igneel - taught magic with the other dragon's element in a more powerful and unique way.
Well, one idea could be the fusion of Naruto's ninja skills with crystal dragon slaying magic. Maybe Naruto discovers a hidden power within him that allows him to summon crystal dragons and use their powers in battles against powerful foes in the Naruto universe.
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.
One idea could be that Lucy discovers a new, rare form of dragon slaying magic. Maybe she finds a hidden dragon lair where this magic is stored and has to overcome various traps and challenges to obtain it.