They might have an epic celebration. The Straw Hats would be excited to see Shanks, especially Luffy. Luffy would probably challenge Shanks to a friendly duel or just be overjoyed to show him how far he's come. Shanks would be proud of Luffy and his crew, and they could all share stories of their adventures over a big feast.
In the fanfiction, there could be a tense moment at first. The Straw Hat Pirates are a rowdy bunch, and Shanks has his own crew. But then they'd realize they're all on the same side. Shanks might offer some advice to Luffy, like how to better handle the power of the straw hat or deal with stronger enemies. And the two crews could exchange treasures or items that are important to them as a sign of friendship.
Luffy would likely run up to Shanks with a big grin and start chatting non - stop about his adventures. Shanks would listen with a smile. The rest of the Straw Hats would be a bit more reserved at first, but then they'd join in. Sanji might offer to cook for everyone, which Shanks would surely appreciate.
One great rival could be a character with a powerful Devil Fruit ability. For example, a character who has a Logia - type Devil Fruit that allows them to turn into a natural element like lightning. This would make them very difficult to defeat in combat as they could dodge attacks easily. They could also use their element - based powers to attack the Straw Hat Pirates in unexpected ways, such as sending bolts of lightning towards their ship.
One popular trope is the 'lost relative' trope. For example, the OC could be a long - lost relative of one of the Straw Hat Pirates. Another is the 'powerful newcomer' trope, where the OC has some extraordinary power that helps the crew in a difficult situation. There's also the 'love interest' trope, although this can be a bit overdone. The OC might fall in love with one of the pirates or vice versa.
First, understand the Straw Hat Pirates' personalities well. For example, Luffy's carefree and determined nature. Then, create an OC (Original Character) that can interact meaningfully with them. Maybe the OC has a special skill or a unique backstory that ties into the pirates' adventures. Think about how the OC can join their crew or be an ally in their journey.
When it comes to navigation, Nami would be interested in Kenway's knowledge of different sea routes and hidden islands. Kenway could learn from Nami about the unique weather patterns in the One Piece world. Also, Chopper would be curious about Kenway's health and any medical knowledge he has from his time as a pirate. Kenway, in turn, would be entertained by Chopper's cute and innocent nature.
In such a fanfic, there might be a lot of interesting character dynamics. The admiral would have to adjust to the Straw Hats' carefree and family - like atmosphere. There could be conflicts at first, as the admiral is used to a more rigid military structure. But over time, they could grow to love and protect their new crewmates just like the others.
They might have an epic battle to test Luffy's growth. Shanks could see how much stronger Luffy has become and be proud. Maybe Luffy will show off his new Haki abilities.
Nami would probably be devastated. She has been with Luffy through so much, and his death would be a huge blow. She might become more withdrawn and serious, as Luffy was often the one to bring out her lighter side.
In the fanfiction, when the Whitebeard Pirates meet Sabo, there could be a show of strength on both sides at first. However, Sabo's connection to Luffy might make the Whitebeard Pirates more friendly towards him. Ace was part of the Whitebeard Pirates, and since Sabo was like a brother to Ace, they could end up having a big celebration in honor of Ace's memory, with Sabo learning more about his time with the pirates.
In a fanfiction where Smoker joins the Straw Hats, it could be really interesting. Smoker is a marine, and his sense of justice is quite different from the Straw Hats' view of freedom. At first, there would likely be a lot of tension. But as time goes on, he might start to understand their way of life. For example, he could see how they help the weak and oppressed in their own unorthodox way. He might also learn to be more flexible in his views of right and wrong, as the Straw Hats often operate outside the normal rules. And his combat skills would be a great addition to the crew, making them even more formidable in battles against stronger enemies.