First, you need to think about their power. It should be powerful but also have some limitations. For example, if your OC can control fire, there should be something that can counter it, like water - based powers or a special type of sea stone. Also, consider their backstory. A tragic past that led them to become a Yonko would make them more relatable.
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.
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.
One interesting concept could be a new Yonko character with a unique power. For example, an OC with the ability to control time in a very different way than what we've seen in the original One Piece. This could create all kinds of new storylines and battles.
In fanfiction, if Luffy becomes a Yonko, he would likely gain more respect and fear from his enemies. His crew might also get more privileges and be involved in bigger power plays. For example, they could have more influence over the territories in the New World.
Luffy would probably be super excited. He's always been on this big adventure, and meeting his dad would be like a new milestone. He might jump up and down and start asking a million questions right away, like what his dad has been doing all this time and if he has any cool stories from his own adventures.
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.