Goku would probably be very friendly with Luffy. They both love to fight strong opponents and have a sense of adventure. Goku might be a bit confused by some of the wacky things in the One Piece world like the Devil Fruits at first, but he'd quickly adapt. He'd also likely be impressed by the determination of the other characters like Zoro and Sanji.
One interesting crossover could be Goku arriving in the One Piece world and teaming up with Luffy to fight a powerful enemy. Goku's incredible strength and combat skills combined with Luffy's Devil Fruit powers would make for an epic battle. Another could be Goku teaching the Straw Hats some of his martial arts techniques, which would enhance their fighting abilities in their own adventures.
Goku Black could be a powerful new threat. His ability to instantaneously move between spaces using his powers would be like a more advanced form of the Shanks' ability to appear suddenly. He could easily target the weak points of the various islands in the One Piece world.
One possible scenario could be Goku Black using his power to take over a part of the One Piece world. He might try to find the One Piece itself, thinking it could grant him even more power. His dark nature would likely put him at odds with the Straw Hat crew, leading to epic battles.
The plot likely involves the adventures of the One Piece characters in a game - specific scenario. It could include exploring new islands, facing off against unique enemies, and perhaps seeking new treasures.
The One Piece Romance Dawn Game likely follows the adventures of the Straw Hat Pirates as in the original One Piece story. It might include battles against powerful foes, exploration of new islands, and the search for the One Piece. Maybe it has unique gameplay mechanics related to Devil Fruits and Haki that players can utilize.
The One Piece game story could center around exploring uncharted islands. Each island has its own unique mysteries, like ancient ruins with hidden treasures or strange creatures. The players, as the Straw Hat Pirates, would need to solve puzzles and fight off enemies to progress.
The main characters are likely to be the members of the Straw Hat Crew, like Luffy, Zoro, Nami, etc. They are the central figures in the One Piece world and would be in the game's original story as well.
One way to develop the characters in a One Piece game story is by putting them in moral dilemmas. For example, Nami might face a situation where she has to choose between saving a village from a corrupt pirate or getting a valuable map that could lead to the One Piece. This would test her character and also allow her to grow in different ways.