A really engaging plot could be set in a post - series world. Gray and Ultear decide to train the next generation of Fairy Tail mages. However, they discover a hidden plot by an old enemy to corrupt the new mages. So they have to not only teach the youngsters but also protect them from this insidious threat. Along the way, Gray and Ultear might develop a deeper understanding of each other's pasts and grow closer as friends.
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 that Ultear and Gray are on a mission together in a mysterious land filled with magical creatures. Along the way, they discover a hidden lemon grove that has some special magical properties related to their powers.
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 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 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.
One idea could be that the Exorcists from D Gray - Man end up in the Fairy Tail world due to a strange magical anomaly. They team up with the Fairy Tail wizards to fight against a common enemy that is causing chaos in both their worlds. For example, a powerful dark mage who has somehow acquired Innocence and is using it in a corrupted way.
One interesting Ultear fanfiction could be about her early days in Grimoire Heart. It might explore her relationships with other members and how she developed her magic.
One interesting plot could be Ultear from Fairy Tail ending up in a different magical realm. She could use her time - magic skills to try and find a way back while getting involved in the local politics and magic disputes there. For example, she might help a group of oppressed mages against a tyrannical magic council.
A plot could revolve around Gray and Erza getting trapped in an alternate dimension. In this dimension, they have to find a way back home while also dealing with their own growing feelings for each other. They might discover secrets about each other's pasts that they never knew before. For example, Gray could learn more about Erza's time in the Tower of Heaven and how it has shaped her into the strong woman she is today. This new understanding deepens their bond.
One idea could be a story where Lucy discovers a powerful magic that can only be controlled with the combined help of Natsu and Gray. So, they have to put aside their usual rivalries and work together closely. This could lead to some really fun and intense training sessions between them as they try to master this new magic.