One common theme is the bond between them. Fanfictions might explore how Ace's relationship with Luffy and Whitebeard affected his decisions and growth. Another theme could be adventures that they might have together that weren't shown in the original manga or anime. For example, they could go on a special mission to a new island.
In fanfiction, their relationships are sometimes explored through shared battles. For instance, they might all fight together against a common enemy, like a powerful navy fleet. During these battles, the trust between Ace, Sabo, Luffy, and the members of Whitebeard's crew is emphasized. The relationships are also shown through quieter moments, like when they are all on the ship sharing stories and dreams, which helps to strengthen the bond between them all.
You can try some popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions related to various anime and manga characters, including Ace, Luffy, and Whitebeard. Another option is FanFiction.net, which also has a decent number of such fanfictions. Just use the search function on these sites and type in the relevant tags or names to find what you're looking for.
Well, perhaps the fanfic could be about how Ace, Sabo, and Luffy's different personalities interact with the Whitebeard pirates. Luffy's carefree nature might bring some chaos and fun to the crew. Ace's loyalty and strength could earn him a special place, and Sabo's intelligence could be used to solve some tricky situations that the crew gets into while sailing under Whitebeard.
Ace's relationship with Whitebeard is already established, but it could grow to be more of a father - son type relationship. Luffy would be in awe of Whitebeard and his crew, and his relationships with everyone would be based on his infectious enthusiasm. Sabo, being the more rational one, would study Whitebeard's leadership style and learn from it. As for the relationship between the three brothers, they would support each other during battles and share their dreams and experiences with the rest of the crew, which would in turn strengthen their bonds with the Whitebeard pirates.
Their relationships would likely develop in a very positive way. Ace already has a connection with Whitebeard. When Sabo and Luffy join, they would be welcomed into the family - like atmosphere of Whitebeard's crew. Luffy, with his infectious enthusiasm, would quickly make friends with the other crew members. Sabo, being more serious - minded, would earn respect for his skills. Ace would act as a sort of bridge between them and the rest of the crew, and over time, they would all become closer, sharing their dreams and fighting side by side.
A plot could revolve around a tournament on an island. Ace, Sabo, Luffy, and members of Whitebeard's crew participate. The prize is a powerful devil fruit. Each character has their own reasons for wanting it. As the tournament progresses, old rivalries are rekindled, new friendships are formed, and they all discover more about themselves and their place in the pirate world.
Their relationship could develop through mutual respect. Luffy respects Whitebeard as a powerful and respected pirate, and Whitebeard respects Luffy's determination and potential. For example, they could start with a simple exchange of stories about their adventures at sea.
Their relationship can be shown as a father - son type. Whitebeard being the protective and wise father figure, always looking out for Ace. For example, when Ace is in a fight, Whitebeard is there to back him up or give him advice based on his own experiences.
One possible scenario in Whitebeard and Luffy fanfiction could be a story where Luffy, after the time skip, meets an older Whitebeard in a spiritual realm. Whitebeard imparts some of his final wisdom to Luffy, about leadership and protecting one's crew. This could be a very emotional encounter that further shapes Luffy's growth as a pirate.