If a marine joins Luffy's crew in a fanfic, it could be quite interesting. At first, there might be a lot of distrust among the crew members. After all, they've been fighting against marines for a long time. But if the marine has a strong sense of justice that aligns with Luffy's own view of right and wrong, they could gradually be accepted. For example, the marine might bring in some new combat techniques or knowledge about the navy's operations that could be useful to the Straw Hats.
They would probably be very surprised at first. After all, they might not have expected to ever meet Luffy's mother. Then they would be excited to learn more about Luffy's past from her.
Most of the crew would be excited to meet the people who brought Luffy into the world. They would see it as an opportunity to understand Luffy better. Chopper might be a bit shy at first but would soon warm up and start asking a million questions about Luffy's early life, like what kind of diseases he had as a kid or how he grew up so strong.
It could put them in a lot of danger. The World Government would target them. But it could also strengthen their bond as they face the challenges together.
Shanks might interact with Luffy's crew in a very friendly and father - like way. He could pat the crew members on the back, give them words of encouragement, and share his experiences with them.
Well, if the Luffy crew were to meet his mother in fanfiction, it could go in many ways. For one, she might be a strong pirate herself. She could then train the crew in new fighting techniques. Or perhaps she has connections all over the world of One Piece and could introduce the crew to important people. It would surely strengthen the bond within the crew as they learn more about Luffy's family.
Fem Luffy would probably bring a lot of energy. She might be more empathetic in some ways compared to the original Luffy, which could help in resolving internal crew conflicts. For example, if two crewmates have a disagreement, she could use her charm and understanding to smooth things over.
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 He would likely become more withdrawn at first, but then focus on getting stronger to avenge Luffy if there was someone to blame for his death.
Obviously, they will support Luffy all the way. He is their captain, and they have complete faith in him. They have been through so much together, and they know Luffy's strength and determination.
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.