A possible plot could involve a dark magic that starts affecting Fiore. Lucy discovers some ancient texts about it, and she realizes she needs the help of Sting and Rogue. They go on a journey to find the source of this dark magic. Along the way, they encounter many enemies. Sting and Rogue use their Dragon Slayer magic to fight, while Lucy summons her Celestial Spirits. As they get closer to the source, they find out that one of their old enemies has returned and is using this dark magic to seek revenge. Together, they combine their powers to defeat the enemy and save Fiore.
Perhaps a story where Lucy, Sting, and Rogue get transported to a parallel dimension. In this dimension, the magic rules are different. Lucy's Celestial Keys don't work as expected, and Sting and Rogue find their Dragon Slayer powers are also affected. They have to rely on each other and their wits to survive. They meet strange versions of their friends in this dimension and have to figure out a way to get back to their own world. During this time, Sting and Rogue start to see Lucy in a new light, and a friendship (or something more) begins to develop between them.
One idea could be a time - travel plot where Lucy, Rogue, and Sting are sent back in time to the founding of Fairy Tail. They have to navigate the early days of the guild while also trying to find a way back to their own time. Another could be a magic - swapping scenario, where they accidentally exchange their magical abilities and have to learn to use each other's magic to fight a new, powerful enemy.
One idea could be that Sting and Rogue initially come to Fairy Tail as spies for Sabertooth, but as they interact with the Fairy Tail members, they start to question their loyalty to their old guild. They could be paired up with different Fairy Tail mages on missions. For example, Sting with Natsu and Rogue with Gray. Through these experiences, they see the true meaning of friendship in Fairy Tail which is different from the competitive environment in Sabertooth.
One idea could be a story where Sting and Rogue have to go on a special mission together in a new, uncharted land. They face various magical creatures and have to learn to trust each other even more. Another could be a fanfic set in an alternate universe where they are not part of Fairy Tail, but are instead traveling rogues who cross paths with characters from Fairy Tail in unexpected ways.
For Lucy, you could develop her character by showing her growing more confident in her magic. Maybe she starts to experiment with new ways to summon her Celestial Spirits or combines their powers in unique ways. With Sting, you can explore his past more deeply. Show how his experiences as a Dragon Slayer have shaped his personality, and have him face some internal conflicts related to his power and identity. As for Rogue, you could develop his relationship with his shadow. Maybe he discovers new abilities within his shadow magic. Also, in the fanfiction, put them all in situations that force them to interact and grow together. For example, a mission where they have to protect a village from a powerful dark mage.
One possibility is that it's a fanfic set in an alternate universe where Sting has a special connection with Lucy. They might discover a new form of magic together. For example, Sting's white dragon slayer magic could combine with Lucy's celestial spirit magic in an unexpected way, leading to a whole new set of adventures.
One idea could be that Lucy and Rogue are brought together by a powerful magic artifact. They have to team up to protect it from an evil force that wants to use it for destruction. Along the way, they start to understand each other's pasts and form a deep bond.
A popular fanfiction idea is that Lucy and Sting are transported to an alternate dimension. In this strange new world, they have to rely solely on each other to survive. They learn to work together in ways they never thought possible. As they search for a way back home, their relationship blossoms into a strong partnership filled with mutual respect and admiration.
One possible plot could be that Rogue somehow gets transported to Lucy's world and they have to figure out how to get him back. During this time, they start to understand each other better. Lucy might learn about Rogue's past and his struggles, and Rogue could see the kindness and strength in Lucy. They could develop a friendship or something more as they face various challenges in the Fairy Tail world.
For Lucy, you could explore her celestial spirit powers more deeply. Maybe she discovers a new and rare spirit that has a special connection to Rogue or Sting's magic. For Rogue, his relationship with his shadow could be further developed. He might learn to use his shadow in new ways, and Lucy and Sting could play a role in that discovery. For Sting, his leadership skills could be tested and improved. They could be put in a situation where Sting has to lead a group that includes Lucy and Rogue, and he has to make tough decisions.
Another common theme is power and growth. Writers often show how Rogue and Sting become stronger over time. For example, they might face a powerful enemy and through that experience, they learn new magic or improve their existing skills. It's about their journey to become more powerful dragonslayers.