You can start by giving the OC a unique backstory. For example, if they were an orphan like Ace, it could create an instant connection. Also, decide on their personality traits. Are they shy and reserved or bold and adventurous? Their skills are important too, like being a great navigator or a skilled swordsman.
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.
Growth is a prevalent theme. As they go through various battles and experiences in the fanfictions, they grow both as individuals and in their relationship. Their understanding of each other and their own abilities develops. Additionally, the theme of brotherhood is strong. They are like brothers in the crew, and this brotherly love is often a central focus in these fanfictions.
In 'One Piece' they are on opposite sides, so in fanfiction, seeing them interact in different scenarios is fascinating. For example, they could be forced to team up against a common enemy. Their different abilities, like Smoker's smoke - based powers and Ace's fire powers, can also lead to some really cool fight scenes and power combinations in the fanfictions.
Adventure is also common. They are pirates after all. Fanfictions might take them on new quests or have them explore uncharted islands together, much like in the original series. For example, they could be searching for a hidden treasure that has a special meaning to their family. It's a way to continue the spirit of adventure that One Piece is known for in a new context.
The popularity also stems from the fact that there are so many unexplored aspects of their relationship in the official story. Fanfiction allows writers to fill in those gaps and create stories that satisfy the fans' curiosity. For example, their early days together or how they would interact in different settings.
To write a believable relationship, first, understand Ace's character. He's brave, loyal, and has a bit of a wild side. Show the reader gradually getting to know these aspects of him. Have them start as acquaintances, maybe the reader is a new crewmate. Then, through shared experiences like battles or helping others, their bond grows. For example, they save each other's lives during a fight with a rival pirate crew.
A plot could be centered around a shared enemy. Suppose there is a new and dangerous pirate who has a grudge against both Ace and Bonney. This pirate targets them separately at first, but then they figure out they are both being hunted. So, they team up to fight back. Along the way, they learn about each other's dreams and motives. Ace's dream of making his father proud and Bonney's dream of becoming the greatest female pirate in the world. This mutual understanding strengthens their bond as they face the common foe.
Well, a common theme is love against all odds. In the One Piece world, there are many obstacles like powerful enemies and different loyalties. So, a story where the reader and Ace's love overcomes these difficulties. Family could also be a theme. Given Ace's complex family background, the reader could become part of his new 'family' and how that dynamic changes their relationship. And of course, the theme of self - discovery. As they interact, both the reader and Ace learn more about themselves and their place in the world, which in turn affects their relationship.
One of the key themes could be loyalty. In a possessive relationship in this fanfiction, Ace and Marco might show extreme loyalty to each other. They could be possessive because they don't want to lose what they have in each other. Another theme might be sacrifice. Given their personalities in One Piece, they might be willing to sacrifice a lot for the other in this possessive dynamic.