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.
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 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.
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.
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 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.
If Lucy never joined Fairy Tail, the friendships and relationships among the characters would form in a very different way. Also, many of the story arcs might not unfold as they do in the original plot.
In the fanfic, Lucy could develop a more adventurous side. Since Rogue is often involved in more dangerous and shadowy things, she might be drawn into that world and have to adapt. For Rogue, being around Lucy could make him more in touch with the positive aspects of the magical world. He might start to question his own darker tendencies and strive to be a better person. This could lead to him making different choices in battles and in his relationships with others.
Well, to find a Fairy Tail fanfiction involving Lucy and Froch, start with checking out dedicated Fairy Tail fan communities. These communities often have their own sections for fanfiction. Maybe post a request there asking if anyone knows of such a fic with Lucy and Froch. Another option is to use Google. Type in something like 'Lucy Froch Fairy Tail fanfiction' and look through the search results. Some of the top results might be what you're looking for.