I'm not sure specifically as there could be many different 'luffy and nami lemon fanfics' out there. But generally, it might involve adventures of Luffy and Nami with some added romantic or fictional elements specific to the fanfic writer's imagination.
Well, it could be that Luffy gets kidnapped because of some secret he holds. Maybe there's a group that wants to find out the location of One Piece through Luffy, and they think kidnapping him is the best way. His friends then have to search for clues and race against time to save him before he's forced to reveal the information.
Well, perhaps Naruto is sent to an alternate dimension where Spartan warriors exist. He gets the Spartan armor there. This armor not only changes his appearance but also his fighting style. He starts to incorporate Spartan combat techniques with his ninja jutsu. He could be in a situation where the two worlds are on the verge of war, and his new look and skills with the Spartan armor make him a key figure in trying to bring peace between the ninjas and the Spartans.
Well, the plot could involve the original character (oc) suddenly finding themselves in Luffy's world. Maybe they are reborn as a new member of the Straw Hat crew. They could have unique abilities from their previous life that help the crew in their adventures, like being able to sense danger more acutely or having a special knowledge of some hidden treasures.
Well, one possible plot could be that after many adventures together, Luffy realizes his deep feelings for Hancock. They might have a big wedding on Amazon Lily with all of Luffy's crewmates attending. Hancock would be overjoyed to finally marry the man she loves, and Luffy would be happy in his own simple way, excited for the new chapter in their relationship and the feasting that would come with the celebration.
One possible plot could be that Luffy gets into a really tough situation during his adventures, perhaps surrounded by powerful enemies on an unknown island. In desperation, he calls out to Garp for help. Garp, despite his tough exterior, races to his grandson's aid. Along the way, he might reminisce about Luffy's growth and how far he has come.
Well, the plot could involve Luffy being taken under Whitebeard's wing after an encounter. Whitebeard, seeing Luffy's potential, decides to train him. Luffy would then be exposed to the different fighting styles of the Whitebeard pirates. He'd probably have sparring matches with the stronger members, like Marco. This would make Luffy even stronger and more well - rounded in his abilities.
Well, perhaps the plot could start with Vivi leaving her kingdom again to join Luffy on his journey for some important reason. As they travel, they face numerous difficulties together and rely on each other more and more. Their friendship gradually turns into love. When Vivi becomes pregnant, it causes a stir among the crew. Some are overjoyed, while others worry about the implications. Luffy, being the captain, has to make decisions about how to keep Vivi safe and still pursue their goals. And there could be a sub - plot where other characters from the One Piece world react to this news, either with envy or with plans to use the situation to their advantage.
Well, one possible plot could be that Luffy starts off as a Celestial Dragon but then has a change of heart due to seeing the cruelty and inequality in the world. He might use his privileged position to start making changes from within the Celestial Dragon system.
One possible plot could be that Female Luffy, with her tenacious and fearless nature, sets out on a journey to challenge Mihawk. She wants to prove herself as a great pirate and swordsman. Along the way, she might encounter various difficulties and meet new friends who help her in her quest. Mihawk, initially seeing her as just another young upstart, gradually comes to respect her determination.