Ace and Luffy are the main draws, of course. However, in many fanfictions, you'll also see Sanji from Luffy's crew getting involved. He can add some comedic relief and his fighting skills are useful. Among the marines, Tashigi is sometimes included. She has an interesting dynamic with the Straw Hat crew, and her sense of justice can create some interesting story elements in the fanfiction.
Obviously, Ace and Luffy are the most popular. But other characters like Zoro from Luffy's crew are also often included. On the marine side, characters like Smoker are quite popular as he has a sort of grudging respect for Luffy. Also, Akainu is often included as the big bad marine that Ace and Luffy have to face off against.
Well, in fanfiction, Luffy's bond with Ace and Sabo is unbreakable. They grew up together and share a lot of memories. When it comes to the Marines, they represent the 'order' that Luffy and his brothers are sort of defying as pirates. Some fanfictions might show a young Marine recruit who has a change of heart after seeing the strong bond between Luffy, Ace, and Sabo. He starts to question whether the Marines are always in the right. Usually, though, the Marines are there to create conflict for the trio.
In fanfiction, a recurring storyline is the Marines' attempts to undermine the brotherly bond of Luffy, Ace, and Sabo. They might try to spread false rumors or create misunderstandings between them. Another popular one is the story of a Marine defector who joins Luffy's crew after realizing the corruption within the Marine ranks. Also, there are stories that focus on the power - up of Luffy, Ace, and Sabo in relation to the Marines. For example, they might gain new abilities and then use them to outwit the Marines in a big battle. There could also be fanfictions that explore the emotional side of the Marines, like a Marine who feels guilty for having to fight against the brothers because he secretly admires their loyalty to each other.
One interesting storyline could be Ace and Luffy teaming up against a powerful marine squadron. They might use their unique abilities, like Ace's fire powers and Luffy's stretchy attacks, to outwit the marines. Another could be a story where Luffy tries to rescue Ace from a high - security marine prison, facing many challenges along the way.
Well, 'Luffy and Ace: Unbreakable Ties' is a fanfiction that I really like. It focuses on their childhood memories and how those shaped their relationship as they grew up. There's also 'Ace and Luffy's Great Escape' which is full of action. It shows their cleverness in getting out of difficult situations. 'Luffy and Ace: The True Friendship' is a must - read too. It simply and effectively shows the depth of their friendship.
The fanfiction 'Ace's Influence' is quite popular. It shows Ace as a very dedicated figure in raising Luffy. He passes on his wisdom and courage to Luffy, which is really inspiring.
Whitebeard is another character that could be important in these fanfictions. As Ace's adoptive father, he plays a significant role in Ace's life. Fanfictions might explore how Whitebeard's influence on Ace affects his relationship with Luffy, like how Whitebeard's values of family are passed on to Ace and then reflected in how he treats Luffy.
One interesting Ace and Luffy fanfiction could be about their adventures in a parallel universe. In this story, they might face different versions of their enemies and have to adapt their powers and strategies. Another could be a story where Ace survives and they continue their journey together to find the One Piece, strengthening their bond even more as brothers.
In this fanfiction, the two central characters are Ace and Luffy. Ace, being the older brother, is likely to be the more protective and guiding one. Luffy, on the other hand, is full of energy and curiosity, always ready for an adventure. Their different personalities will surely lead to many interesting events in the story.
Well, in this fanfiction, Luffy could interact with the Marines in a more diplomatic way. Since he has the advantage of time travel, he might understand the reasons behind the Marines' actions better. So, he could try to negotiate or make deals with some of the more reasonable Marines. He could offer to help them with other threats in the One Piece world in exchange for them leaving his friends alone at Marineford.
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.