Maybe they find a strange fruit that has the power to temporarily swap their abilities. This leads to some comical and also interesting situations. Luffy tries to use Robin's powers to reach for things in a more elegant way, and Robin attempts to use Luffy's stretching ability to explore hard - to - reach places. It also makes them understand each other's perspectives better.
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.
Another idea is that they are separated from the rest of the Straw Hat crew. In this situation, Luffy and Robin rely on each other more than ever. They travel through a dangerous area filled with strange creatures. Luffy protects Robin from the physical threats, and Robin uses her skills to find food and shelter. Along the way, they have deep conversations that strengthen their bond as nakama.
Maybe in the fanfiction, a smart Luffy figures out a way to read Poneglyphs with Robin's help. This new ability draws the attention of both the World Government and other pirate crews. They then have to go on a journey to protect this new - found knowledge while also using it to uncover more secrets about the Void Century.
One idea could be Luffy accidentally eating a Devil Fruit that gives him the ability to create lemon - scented mirages. He uses this to fool his enemies during battles. Another could be a story where Luffy discovers a hidden island full of lemon - based treasures that are said to have special powers. And perhaps a plot where Luffy teams up with a lemon - loving character who has a unique fighting style that involves using lemons as weapons.
One idea could be that Luffy accidentally eats a devil fruit that makes him see Boa Hancock in a new light. He starts to notice her charm in a more romantic way, but still retains his innocent and goofy nature. This creates a series of comical and heart - warming moments as Boa Hancock tries to understand his new behavior.
One idea could be that during a dangerous adventure, Zoro and Robin get separated from the crew in a mysterious island. There, they have to rely on each other for survival. As they face various challenges together, their bond deepens and a romantic relationship starts to form, filled with their unique personalities interacting. For example, Zoro's tough and straightforward nature combined with Robin's calm and intelligent demeanor.
Since the 'lemon' part makes it a bit ambiguous, but if it's a regular fanfiction about Luffy and Makino, it might explore their relationship in the context of Luffy's journey. For example, Makino could be a source of support for Luffy in a more emotional way, like when he has doubts about his role as a pirate. The fanfiction could show how their interactions develop over time and how they influence each other's lives.
One interesting plot idea could be that they go on a special adventure to find a lost ancient island that only Robin can decipher the clues to reach. And along the way, Luffy's unwavering determination and Robin's knowledge work together to overcome various obstacles like strange sea creatures and difficult terrains.
One idea could be that Robin and Artemis are on a solo mission together and get trapped in a strange, magical place. As they try to find a way out, their feelings for each other intensify. Another could be a post - battle scenario where they're both injured and have to rely on each other for care, leading to some romantic moments. Or perhaps a time - travel plot where they see alternate versions of themselves in different relationships and it makes them realize their feelings for one another.
I'm not sure about the specific 'luffy x reiju lemon fanfiction' but usually, in fanfictions, the plot could involve their unique interactions, adventures together, or exploring their relationship in ways not shown in the original story.