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.
The unique aspect is that it combines the familiar characters of One Piece with the concept of reaction. It's fun to see how characters like Luffy or Zoro would react to things they might not have known before, like modern technology or different interpretations of their own stories.
Some would be excited. Luffy would probably be eager to jump into the future he sees in the fanfiction. He's always looking forward to adventures, and if the future seems cool, he'd be all for it.
Sure. 'One Piece: The Straw Hats React to Their Future' is a great one. It shows how the Straw Hat crew reacts to seeing possible future events of their adventures, which is really interesting.
They may laugh out loud if there are humorous parts in the fanfic. For instance, if the fanfic writer creates a comical situation involving the characters that is well - written.
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.
Luffy would be really emotional. He might laugh at the funny parts of Ace's past and cry at the sad ones. He loves his brother so much that any story about Ace would have a big impact on him.
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.