Perhaps a plot where she gets lost in a mysterious magical realm and Natsu has to search for her. Along the way, she discovers her own strength and makes new friends in the realm. It could be filled with challenges and self - discovery.
One interesting plot could be her going on adventures with Natsu and his friends. She might have unique magical abilities that help the group in their quests.
One interesting plot could be Hisoka's sister getting involved in the Hunter Exam. She might use her unique skills, perhaps some that are similar to Hisoka's but with her own twist, to navigate through the challenges. For example, she could have a special ability to read opponents' intentions like Hisoka, but she uses it in a more empathetic way to avoid fights when possible.
One interesting plot could be that Gaara's little sister has a special power that she has to keep hidden from the village at first. This creates a lot of mystery and tension as she tries to navigate her life without being discovered.
There could be original characters created specifically for the fanfiction. For example, a magical creature that befriends Natsu's little sister and helps her on her journey. But the core Fairy Tail characters usually play important roles as well. They are the ones who provide the familiar backdrop of the Fairy Tail world that the story is set in.
One plot idea could be that Natsu's little sister discovers a new and powerful form of magic that no one has ever seen before. She then has to learn to control it while also facing off against an evil wizard who wants to steal her power for himself. Another idea is that she goes on a journey to find out more about their family's past, which leads her to uncover long - lost secrets and reunite with long - lost relatives.
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.
She could also be a key figure in the political intrigue of Camelot. As Arthur's sister, she has a certain status. There might be those who try to use her to get to Arthur or undermine his rule. So she has to navigate the dangerous world of court politics, using her wits and charm to outwit her enemies and protect Arthur and Camelot.
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.
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.
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 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.