Maybe a Truth or Dare game that is magically enhanced. So, when Natsu or Lucy answers a truth or does a dare, it has some unexpected magical consequences. For example, if Natsu is dared to hold Lucy's hand for an hour, it creates a magical bond between them that gives them new powers.
A plot could involve a rival guild challenging Fairy Tail to a Truth or Dare battle. Natsu and Lucy are chosen as the main participants. As the game progresses, they not only have to face the challenges but also deal with their growing feelings for each other. The dares could be things like Natsu having to protect Lucy from a series of magically created threats while Lucy has to use her wits to answer some really tough truths about Natsu's past.
One idea could be that during a Truth or Dare game at the guild, Natsu is dared to confess his true feelings for Lucy. This could lead to a whole new level of their relationship development.
One interesting plot idea could be that in the genderbend world, Natsu as a girl is trying to prove herself in the Fairy Tail guild which is still male - dominated in terms of leadership. She wants to break the glass ceiling and show that she can be as strong as any male wizard. Another could be that Lucy as a boy has a different approach to celestial magic. Maybe he is more brash and uses the keys in a more combat - oriented way rather than the more diplomatic or emotional way Lucy does. And a third idea is that their relationship dynamics change completely. The girl Natsu might be more shy about her feelings towards boy Lucy, which is a complete reversal of their normal dynamic.
Well, perhaps in this office fanfiction, the office is actually a front for a secret magical organization. Natsu and Lucy are brought in not knowing the real nature of the place. As they start to uncover the truth, they find themselves in the middle of a power struggle between different factions within the organization. Their relationship grows as they rely on each other to survive and expose the corruption within the organization.
One idea could be the children of Natsu and Lucy having to deal with a new dark guild that has emerged. They inherit their parents' powers but also have to figure out how to use them in new ways to combat this threat.
Perhaps the future child is born with a special mission. In the fanfiction, it could be that the child has to reunite different factions of the Fairy Tail guild that have split apart in the future. This would involve the child using their knowledge of their parents' friendship and the power of Fairy Tail's bonds. Also, the future child might face an enemy that is related to an old foe of Fairy Tail, and they need to draw on their family's legacy to defeat it.
One idea could be that Natsu and Lucy discover they have a long - lost family connection that makes them siblings. They then have to deal with the new dynamic while still being part of the Fairy Tail guild. For example, Natsu might become overly protective in a different way than before.
One idea could be that Natsu finds an ancient Fire Dragon spell that creates the most beautiful and unique wedding venue filled with fire - like flowers. Another is that Lucy writes a special story about their journey together as part of her wedding vows. Also, all of their Fairy Tail friends could contribute their magic to create a spectacular display during the ceremony.
One idea could be that the Nalu twins have special powers that are a combination of Natsu and Lucy's magic. For example, they can use fire magic like Natsu but also summon celestial spirits like Lucy. Another idea is that they get into adventures in the Fairy Tail guild, following in their parents' footsteps and facing challenges that test their friendship and their abilities.
Some Fairy Tail fan groups on Facebook might also be helpful. Members often discuss and share their favorite fanfictions. You can join a few groups, post a request for Nalu Truth or Dare fanfictions, and see what others recommend.
One idea could be that Natsu and Lucy are transported to a world similar to that of Black Bird, where they have to face new and strange magic. They might encounter characters from Black Bird who mistake them for enemies at first. Another plot could involve a powerful dark magic that is affecting both the Fairy Tail world and the Black Bird world, and Natsu and Lucy, with their unique bond, have to find a way to stop it.
One idea could be that Natsu accidentally uses a new spell that creates an intense heat wave around him and Lucy has to find a way to cool him down while also dealing with the chaos it causes in the guild. Another could be a forced heat - related spell that makes Natsu and Lucy work together more closely than ever to defeat an enemy who is using cold - based attacks.