In some fanfictions, Lucy and Gray could leave Fairy Tail due to a prophecy. They believe that their presence in the guild will bring danger to their friends. So, they leave secretly. On their journey, they discover secrets about their own powers and pasts that they never knew before. They also encounter old enemies who are now after them for leaving Fairy Tail as it disrupts the balance of power that was maintained in the guild's territory.
Perhaps some dark mages who have a vendetta against Fairy Tail. They target Lucy and Gray because they think that these two are easier to take down outside of the protection of the guild. They might want to use Lucy and Gray as bait to draw out other members of Fairy Tail.
There could also be a plotline where they both discover a new magic together. While exploring this new magic, they have to train closely and that physical and mental closeness could spark a romantic relationship. Maybe it's a long - lost ice - and - celestial - combined magic that only they can master, and as they figure it out, their relationship evolves.
Often, there could be a plotline where Gray is forced into a situation that makes it look like he's cheating. Say, an enemy of Fairy Tail frames him by creating false scenarios. Or it could be a situation where Gray is trying to protect Lucy from a greater danger by getting closer to someone else, but Lucy doesn't understand his intentions at first. There are also plotlines where Gray's past comes back to haunt him and his interactions with an old acquaintance seem like cheating to Lucy.
Perhaps they leave because they feel underappreciated in Fairy Tail. They might think that their skills and contributions are not being recognized enough, so they decide to go off on their own to prove their worth elsewhere. They could start their own small guild or join another existing one that values them more.
A plotline could also involve Lucy having to team up with an unlikely group of characters, perhaps some former enemies or characters from other guilds. Together, they have to face a common threat that is much larger than any of them could handle alone. This forces Lucy to use her wits and her magic in new and creative ways, and also develops her relationships with these new allies in unexpected ways.
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.
One interesting plotline could be a magic - based adventure where Erza and Lucy team up against a powerful dark mage. They might use their unique magical abilities in tandem, like Erza's requip magic and Lucy's celestial spirit magic to overcome challenges. For example, they could be on a quest to retrieve a powerful magical artifact that has been stolen by the dark mage. Along the way, they strengthen their friendship and learn new magical techniques from each other.
One great Gray and Lucy fanfiction is 'A New Bond'. It focuses on their adventure together in a mysterious land, where they have to rely on each other's skills. Gray's ice - making magic and Lucy's celestial spirits work in harmony to overcome challenges.
Another common plot is Gray getting jealous when Lucy is getting too much attention from other guys in the guild. This jealousy leads to some really interesting interactions between them, like Gray trying to show off in front of Lucy in a really cute and clumsy way.
A possible storyline is a friendly competition between them. They bet on who can complete a series of challenges first. These challenges could involve things like rescuing a certain number of people in a village within a time limit, or collecting rare magical items. Along the way, they learn more about each other's strengths and weaknesses, and also grow closer as friends. This kind of storyline can also have some humorous moments, like when Natsu accidentally sets something on fire during a task that Gray then has to quickly freeze to prevent a disaster.