They might be shocked. Luffy has so many wild and dangerous adventures in his memories. His friends would probably be in awe of how much he has endured and how strong his will is.
The characters' reactions can be quite diverse. Chopper would be extremely worried and immediately start his medical examination. He'd be constantly checking Luffy's symptoms and trying different remedies. Nami would be worried too but also thinking about how this could affect their journey and the crew's safety. Zoro might seem nonchalant on the outside but would be secretly on high alert, ready to fight off any danger. Sanji would cook up all kinds of nutritious food to help Luffy get better.
They might be shocked. Luffy's Gear 5 is such a powerful and strange form. In the fanfiction, characters could be amazed at his new abilities, like his cartoon - like movements and seemingly limitless power.
One Piece Luffy and Nami were one of the main characters in One Piece. Luffy was a brave and adventurous boy. His dream was to become One Piece, find the legendary One Piece, and become the most powerful pirate in the world. As for Nami, she was a female character with the ability of a genius navigator. Her dream was to find her " Goddess of the Sea " Robin and protect her from any danger.
Luffy and Nami played a very important role in the story, and their relationship was very rich. In the story, Luffy met Nami in an adventure and began their adventure. Although they often faced various dangers and difficulties, they had always supported and encouraged each other, becoming inseparable partners.
I think Shanks would have an interesting reaction. He has known Luffy since he was a kid. Seeing Luffy reach Gear 5, he would probably be proud but also a bit cautious. He might think about how far Luffy has come and also wonder what challenges this new power will bring for Luffy and the world at large.
In a One Piece fanfic, an OC (Original Character) as Luffy's rival could be really interesting. This OC might have a unique Devil Fruit ability that directly opposes Luffy's powers. For example, if Luffy has the Gum - Gum Fruit which gives him stretchy abilities, the rival could have a hardening or shrinking ability. This would create intense battles between them as they fight for different ideals on the Grand Line.
One popular type of 'One Piece' fanfiction involving Luffy and Dragon might be about their eventual reunion. Since Dragon is Luffy's father, many fans like to imagine the moment when they meet again. In some fanfictions, Dragon could be secretly guiding Luffy's journey without him knowing. This adds an interesting layer of mystery to Luffy's adventures as he is unknowingly following a path his father has some influence over.
One possible fanfiction could be about Zoro and Luffy's unwavering loyalty to each other. Zoro is always there to protect Luffy and vice versa. They might face a powerful enemy that tests their bond, and through their combined strength and trust, they emerge victorious.
If Luffy was kidnapped in a fanfiction, his crew would surely go to great lengths to rescue him. Nami would probably come up with a smart plan, using her knowledge of the seas and navigation. Zoro would be ready for a fight, not fearing any opponents in the way. Sanji would keep everyone fed during the rescue mission, while also being prepared to kick some kidnapper butts with his powerful kicks.