In some fanfictions, Natsu could be kidnapped because of a misunderstanding. Maybe another group thought he was causing trouble in their territory when in fact he was just passing through. Gray would then have to prove Natsu's innocence while also getting him out of the kidnappers' clutches. This could involve some intense battles and negotiations.
One possible plot could be that the kidnappers are from a rival guild who want to use Natsu as leverage against Fairy Tail. Gray could then embark on a solo mission to rescue Natsu, facing various traps and enemies along the way. He might have to use his ice - make magic in creative ways to overcome obstacles.
Gray would be extremely angry. He would immediately start looking for clues as to where Natsu was taken. His ice - make magic would be on full display as he might freeze things in frustration while trying to figure out the situation.
Well, a plot could involve Natsu being completely oblivious to Mirajane's jealousy. He might be so focused on his own goals, like defeating a powerful enemy or achieving a certain magical feat, that he doesn't notice how his actions are making Mirajane feel. Meanwhile, Mirajane is seething with jealousy and the other guild members start to notice the tension between them, which could lead to some interesting interactions and attempts at mediation by their friends.
One idea could be that in a world where Natsu is female, she and Gray are paired on a difficult mission. They have to work together to defeat a powerful dark wizard. Along the way, they start to understand each other's fighting styles better and develop a mutual respect that turns into a budding friendship. Maybe Natsu's more emotional side as a female adds a new dynamic to their interactions.
There's a fanfiction where Natsu and Gray accidentally switch bodies. It's really funny as they try to act like each other in front of the guild members. Natsu in Gray's body has a hard time controlling the ice magic and Gray in Natsu's body keeps accidentally setting things on fire. But through this chaos, they gain a deeper understanding of each other.
The contrast between their personalities. Female Natsu, with her bold and energetic nature, and Gray, with his more reserved and cool self, creates a dynamic that can lead to very interesting interactions and storylines.
One interesting plot could be their journey of getting to know each other on a deeper level before the marriage. Maybe they go on adventures together and face challenges that strengthen their bond. Another could be how their friends in the guild react to their relationship and upcoming marriage, with some being overly excited and others being a bit more cautious.
Another plot could be a story set in a different time period. They are transported to the past and have to adapt to the old ways while also trying to find a way back to their own time. In this journey, they rely on each other a great deal and their bond becomes unbreakable.
Another plot could be centered around their fears and hopes. Lucy might be worried about being a good mother, considering her own upbringing. Natsu, on the other hand, could be full of hope that their child will be as strong as him, but also a bit scared that he won't be a good enough father. He then decides to go on a journey to learn from other dads in the guild.
In this fanfiction, a possible plot is that Natsu has to protect his friends from those who fear his new power as a dragon god. His friends initially might be a bit wary of him too. But as the story progresses, he shows them that he's still the same Natsu at heart. He uses his powers to save the guild from a great danger, like an invasion from a powerful dark force that only his dragon god powers can defeat.
One popular theme is their rivalry. They are always competing with each other in the series, and in fanfiction, this can be explored more deeply, like who can complete a mission faster or who is stronger in a new magic ability.