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.
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.
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.
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.
Mobius 1 is often very popular. He's the protagonist in many of the main games and his skills and mysterious nature make him a great character for fanfiction. Another popular one is Pixy. His complex relationship with Mobius 1 and his own moral struggles are fascinating to explore in fan - created stories. And Cipher is also quite popular as he has a lot of mystery surrounding his actions and identity in the game.
Obviously, Luffy himself is extremely popular. His carefree and fearless nature makes for great stories in the gaming context. For example, he might charge headfirst into dangerous in - game situations without hesitation.
Some fanfictions also pair Luffy with Hancock. Given Hancock's infatuation with Luffy in the original series, it's not a stretch to develop a full - blown love story. Her powerful status as a pirate empress and Luffy's carefree nature can create an interesting dynamic, with Hancock trying to win Luffy's heart while he is mostly oblivious to her romantic intentions.
Mobius 1 is also highly popular. Given his reputation as a skilled pilot in the Ace Combat series, he can be easily incorporated into Ace Combat 3 fanfiction. Writers can create scenarios where he teams up with the existing characters in Ace Combat 3 or has some sort of rivalry with them. And then there's Pixy. His complex personality, especially his views on war and loyalty, can be a great source of inspiration for fanfiction writers.
There's an OC Marine who is known for his diplomatic skills as well as his combat abilities. This character is able to bridge the gap between different species in tense situations, which makes him stand out. And then there's the female OC Marine who is a brilliant strategist. She's often involved in planning complex military operations that save the day in many fanfictions.
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.