First, you need to establish the backstory of the OC rival. Why do they see the Straw Hats as their enemy? For example, if the OC was once wronged by a member of the Straw Hats accidentally. Then, think about the OC's abilities and how they can challenge the Straw Hats. Maybe the OC has a power that can nullify Luffy's rubber abilities for a short time. As the story progresses, the OC can gradually gain respect for the Straw Hats through different encounters, like when they team up against a common enemy. This could lead to a change in the OC's attitude towards them.
Well, for an OC rival, let's call him Nova. Nova is a pirate who has been around longer than the Straw Hats. He has a crew that is very powerful. Nova's motivation to be their rival is that he wants to be the most famous pirate in the world, and he sees the Straw Hats as a threat. He's a master strategist and has a crew with various special abilities. For example, his first mate can create illusions, which makes it difficult for the Straw Hats to fight them in a straightforward way.
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.
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.
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.
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.
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.
Zoro would be deeply affected. He has sworn to follow Luffy and his death would be a huge blow. He might feel like he failed in his duty to protect the captain. He would likely become more withdrawn at first, but then focus on getting stronger to avenge Luffy if there was someone to blame for his death.
Luffy and Zoro's bond would still be strong. Maybe in a reincarnated life, they meet as rivals at first in a martial arts tournament but soon realize they share the same goals and become partners again. Nami and Usopp could be siblings in their new lives, with Nami being the protective older sister and Usopp using his creativity in a new family dynamic.
Luffy would be furious. He'd immediately set out to rescue Sanji without a second thought. He'd probably charge straight at Doflamingo's stronghold without a proper plan at first, just driven by his anger and determination to get his crewmate back.