Their relationship could develop through a shared love for a particular type of magic. For example, if they both have an affinity for celestial magic (even if Rogue in a different way), they could bond over researching and exploring new aspects of it. This would give them a common ground and something to always talk about.
Another way to develop their relationship uniquely is by having them be the only ones who can understand a certain ancient magic language. They are brought together to translate a powerful spellbook. As they spend time deciphering the text, they learn about each other's dreams, fears, and hopes. This shared intellectual pursuit could lead to a very deep and special connection between Lucy and Rogue.
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.
Their relationship could develop through a shared love for a certain type of magic. For example, if they both find an ancient and powerful magic that only they seem to understand, it could bring them closer. They start sharing their knowledge and experiences related to this magic, which deepens their bond.
Their relationship could be developed uniquely by having Lisanna teach Natsu some of the more delicate magic she learned in Edolas. This would create a role - reversal situation where Natsu is the one learning from her. It would show a different side of their dynamic and deepen their connection as they spend time in this new kind of interaction.
Their relationship could be developed uniquely by having them start as rivals in a magical competition within the guild. As they compete, they learn about each other's strengths and weaknesses, and a mutual respect forms. This respect then turns into an unexpected friendship that eventually grows into love.
Their relationship could be developed in a unique way if they had to swap magic powers for a short period. Gray having to use Erza's requip magic and Erza using Gray's ice - magic would force them to understand each other on a deeper level. They would have to train together to master the new powers, which would lead to a lot of close - quarter interactions and shared experiences.
Their relationship can be developed through shared experiences. For example, if they are on a mission together and have to rely on each other to survive. This can create a sense of trust and respect between them.
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.
The relationship can be developed through shared experiences. For example, if they are both involved in rescuing a kidnapped guild member. They'll face danger together, and Freed might use his magic to protect Lucy from harm, which will make Lucy see him in a new light. This will create a sense of trust between them.
Lucy could give Aquarius more freedom outside of the key. Let her experience the human world more often. They could go on adventures that are not related to Fairy Tail's main quests, just the two of them. This would allow them to get to know each other on a more personal level. Aquarius could also teach Lucy more about celestial magic and its deeper secrets, which would bring them closer as they explore this knowledge together.
A unique celebration could be a celestial spirit - themed party. The guild could transform into a starry sky - like setting. The celestial spirits themselves could come down and perform a special dance for Lucy. And they could all share celestial - themed food and drinks, like star - shaped cookies and moon - juice.