One idea could be that Law has a very specific reason related to a grand plan against the World Government. He believes Luffy's unique abilities or connections can help, so he 'kidnaps' him in a rather non - malicious way at first. Maybe he wants to use Luffy's Gear Fourth power for some kind of experiment or strategy.
Nami could be a main character. She would be very worried about Luffy and try to find him. She might use her navigation skills to track down Law's ship. And she could be the one to try and negotiate with Law for Luffy's release.
One interesting plot idea could be that Luffy, in her female form, discovers that her Mythical Zoan powers have a special connection to Law's devil fruit powers. Maybe they accidentally trigger a combined power that creates a unique adventure for them. They could go on a journey to find a way to control this new power while also exploring their growing feelings for each other. For example, during battles, they find that they can combine their abilities in unexpected ways, which helps them defeat stronger enemies and brings them closer together.
In this type of fanfiction, it might be that Harry has been misled by false prophecies. He kidnaps Hermione thinking he is doing the right thing. As the story unfolds, Hermione uses her intelligence to make Harry see the truth. They then team up to face the real threat that was behind the false prophecies all along. It could also involve a lot of emotional turmoil between the two characters as they deal with the situation, with their friendship being put to the test.
A plot could involve a character who is forced into a marriage due to some archaic part of the marriage law. The story could be about their struggle to break free and change the law for others in the same situation. They might meet an attorney who is also an advocate for marriage law reform and together they start a campaign. This would involve researching different cases, gathering evidence of the law's negative impacts, and presenting it to lawmakers. There could be a lot of drama as the character's family and the establishment try to stop them from making changes.
A possible plot is that Fem Luffy accidentally gets involved in one of Smoker's investigations in a new island. She initially seems like a hindrance to him, but as the story progresses, they find out that there is a much larger conspiracy at play that involves both the Marines and some powerful pirate groups. They have to work together to expose the truth, and during this time, a strange friendship starts to form between them.
Well, a plot could revolve around Luffy as a girl having to deal with the expectations of femininity in different islands they visit. Some islands might have strict gender roles, and she has to find a way to balance being true to herself as a fearless pirate while also navigating these cultural differences. And perhaps her crew helps her in different ways to overcome these obstacles.
One idea could be that Luffy accidentally stumbles into a parallel world full of female characters who are all smitten with him. Another could be that in a new island they visit, there are a group of girls who have unique powers and they all want Luffy to be their leader and partner. Maybe there's also a plot where a magic spell makes all the female pirates in the Grand Line be attracted to Luffy and they keep following his ship.
Perhaps in this fanfiction, Luffy is transported to a realm where he has to prove himself as the God of War. He could face challenges that test not only his physical strength but also his leadership and strategic thinking, which are also important aspects of being a God of War.
One idea could be that Dragon is on a secret mission when he accidentally crosses paths with Luffy. Luffy, being his usual curious self, wants to know more about this mysterious man who seems so powerful. Another plot could involve Dragon saving Luffy from a powerful and unexpected enemy, and in the process, they start to bond over their shared sense of justice. Maybe Luffy could also learn some new skills or gain some valuable knowledge from Dragon during their encounter.
Well, they could be in a situation where a powerful enemy targets female Luffy specifically because of her unique abilities as a female version of Luffy. Zoro vows to protect her at all costs. This could lead to some intense battles where Zoro has to fight opponents far stronger than him while trying to keep female Luffy safe. Meanwhile, female Luffy could be using her smarts and powers to find a way to defeat the enemy from the sidelines, perhaps by finding its weakness and relaying it to Zoro during the fight.