Gray could find himself in a time - loop where he has to keep reliving the same day over and over again. Each day, a Devil gets stronger and closer to destroying Fairy Tail. He has to figure out how to break the loop and defeat the Devil once and for all. This could involve him exploring different relationships with his fellow guild members and using their unique skills in new ways to solve the problem.
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.
One interesting character could be a Devil Slayer who has the power to manipulate shadows. He can use the darkness not only to attack demons but also to move stealthily. His backstory of how he got his powers, perhaps from a fateful encounter with a shadow - based demon, makes him really engaging.
One idea could be a story where Gray and Ultear team up to face a new, powerful dark magic user. They have to combine their ice and time - magic skills to defeat the enemy. Another plot could be a more character - driven one, like Ultear trying to help Gray overcome some past traumas related to his family and Deliora.
One idea could be exploring Gray's past more deeply. Maybe write about his training days with Ur and the challenges he faced. Another could be a what - if scenario where Gray joins a different guild for a time. For example, how he would adapt to their rules and make new friends or enemies there.
One idea could be a slayer from a different dimension coming to the Fairy Tail world. This new slayer could have unique powers that no one in Fairy Tail has seen before, leading to all kinds of adventures and challenges as the guild tries to figure out how to deal with this new arrival.
Gray can be developed further by exploring his relationship with his Devil Slayer powers on a deeper level. Maybe he starts to have dreams or visions related to the origin of Devil Slaying magic, which makes him question his role. He could also develop by having more in - depth interactions with other Devil Slayers outside of Fairy Tail. This would expose him to different ways of using the power and different moral perspectives.
One idea could be a story where Juvia and Gray get trapped in a magical realm together. In this realm, their powers are altered in strange ways, and they have to rely on each other more than ever. They start to discover new aspects of their relationship as they face various challenges.
One idea could be a story where Juvia, Lyon, and Gray have to team up to face a powerful dark wizard that is threatening the Fairy Tail guild. They start off with some tension due to their past relationships, especially between Gray and Lyon who have a rivalry, but as they battle, they learn to work together better. Juvia could also be in a position where she has to choose between protecting Gray or following Lyon's more strategic plans, adding an interesting love - triangle - like dynamic to the plot.
Another plot could revolve around the origin of Dragon Slayer magic. Maybe a long - lost scroll is found that reveals secrets about how the magic was first created and what its true purpose was. This could lead the Dragon Slayers on a quest to find out more about their powers and their connection to the dragons.
Maybe the fanfiction could focus on a dragon slayer who loses their powers but then discovers a new way to regain them. For example, they might have to go on a spiritual journey and face their inner demons. Along the way, they form new bonds with the members of Fairy Tail, which helps them in their quest.
One idea could be the Ice God Slayer having to face an ancient ice demon that was sealed away long ago. As the slayer battles, they discover that the demon has a connection to their past and the origin of their ice - slaying magic.