A possible plot could be that Female Luffy accidentally eats a Devil Fruit that gives her the ability to communicate with animals. Ace is initially skeptical but then realizes how useful this can be. They then go on a journey to find a hidden forest where there are ancient animals that are said to know the location of a great treasure. Along the way, they face challenges from other pirates who also want to get to the treasure first, and they have to use their combined skills and Luffy's new ability to outwit their opponents.
One idea could be that Female Luffy and Ace team up for a special treasure hunt. They might discover a long - lost island that holds a powerful Devil Fruit that has unique abilities. Another plot could involve them facing a new and powerful enemy together, and during the battle, they learn more about each other's dreams and pasts. For example, they could find out about a connection between their families that they were previously unaware of.
One idea could be Ace crossing over into a modern - day world where he has to adapt to new technologies while still having his devil fruit powers. He might get involved in some sort of street racing using his fire powers to boost his vehicle.
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 Ace traveling back to the time when Gol D. Roger was still alive. He could witness the events that led to Roger's surrender and execution, and perhaps even interact with the Pirate King. This would give him new insights into the world of piracy and his own place in it.
A great plot idea could be that the OC is a healer who saves Ace's life after a particularly brutal fight. Ace is initially distrustful but as he recovers, he sees the kindness in the OC. They then go on a journey to find a rare herb that can cure a disease that is spreading among the pirates, and along the way, their relationship blossoms.
One idea could be that Shanks teaches Luffy a special, secret Haki technique that only they share. This would give Luffy an edge in his future battles and also strengthen their bond.
One idea could be that Hancock's feelings for Luffy lead her to secretly help the Straw Hats in a dangerous situation. For example, when they are facing a powerful enemy on an island, Hancock uses her influence and powers to divert the enemy's attention.
In some One Piece Luffy Gear 5 fanfiction, there could be the idea of Luffy sharing his Gear 5 power with his crewmates temporarily. This would not only make for some cool team - up attacks but also show the deep bond within the Straw Hat crew. Also, a fanfiction might explore how Luffy's Gear 5 affects his relationship with other pirates or the World Government. Maybe it makes him an even bigger target or maybe it causes some pirates to want to ally with him out of fear or respect.
One idea could be exploring how Luffy's unique rubber body affects his metabolism. Since his body can stretch, perhaps it requires more energy to maintain that elasticity, thus having a super - fast metabolism. Another idea is to incorporate Ace's fire - based abilities and how that might interact with Luffy's metabolism in a fanfiction scenario. Maybe the heat from Ace's powers could speed up Luffy's metabolism even more in a fight or during a challenging situation.
Perhaps in a fanfiction, Bonney could be a stowaway on the Thousand Sunny. Luffy discovers her and instead of being angry, he welcomes her as part of the crew. Then they go on adventures together, with Bonney learning about Luffy's dream and starting to believe in it too.
A common theme is also the exploration of their physical attraction. Writers might describe how they notice each other's bodies in a different way than just the normal brotherly concern. For instance, Ace might be captivated by Luffy's carefree nature and how it's reflected in his movements and expressions, which then leads to a physical attraction. This can be a starting point for a more in - depth romantic relationship in the fanfiction. There could also be themes around jealousy, like if another character shows interest in Luffy, Ace would feel jealous in a romantic way.