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.
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.
First, know the One Piece characters really well. Their personalities, dreams, and relationships are key. For example, Luffy's carefree but determined nature should be accurately portrayed when he reacts to something.
One popular 'One Piece reaction fanfic' is the one where the Straw Hat crew reacts to seeing their future selves. It's interesting to see how they would handle the knowledge of their potential growth and the challenges they will face.
Well, there are fanfictions where the One Piece characters react to modern - day Earth things. For example, Luffy's reaction to a smartphone would be hilarious. He'd probably try to eat it at first, thinking it was some kind of strange fruit. And then there are those where the crew reacts to other anime characters showing up in their world. It creates really unique and fun crossovers.
Well, Ruby would be all eyes and ears, loving the new adventures presented in the fanfiction. Weiss might initially scoff but then get drawn in, especially if there's some complex plot. Blake would look for deeper meanings and character development, just like she does in her own story. Yang would cheer for the characters in the fanfiction as if they were real, and might even get some new ideas for her own fighting style from the epic battles described.
First, decide which characters you want to feature in the reaction. It could be the whole Straw Hat crew or just a few of them. Then, think about what they are going to react to. It could be a new Devil Fruit, a powerful enemy, or a strange new island. After that, start writing their initial reactions and build on it.