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.
If Goku were to interact with the One Piece characters, he would be very straightforward. For example, when meeting Nami, he might not understand her love for money right away but would respect her skills as a navigator. With Usopp, he'd be interested in his inventions. And when it comes to battles, Goku would jump right in to help them against any powerful foes. He'd be like a new and extremely powerful crewmate, sharing his Saiyan knowledge and training methods with them.
Goku's interaction with One Piece characters would be quite fascinating. He has a pure heart and loves a good fight. So, when he meets the Straw Hats, he would immediately bond with Luffy over their shared love for combat. He would probably find Chopper's transformations interesting and might even try to figure out if there's a way to use his Ki to enhance Chopper's forms. Goku would also be in awe of Franky's inventions and might offer some ideas from his own experiences in building things in his world. His presence would definitely shake up the dynamics of the One Piece crew, bringing in a whole new level of power and an alien (to them) way of thinking.
In this fanfiction, aside from Gohan, there could be other characters. For instance, Piccolo. Piccolo might sense Goku's power and be both intrigued and a bit wary. He could approach baby Goku to either assess the threat or try to understand his power source. Also, Chi - Chi might be involved. Even as a child herself, she could be drawn to Goku because of his unique presence and power, which could start their early relationship in a different way compared to the original story.
Well, a Goku - One Piece crossover fanfiction might involve Goku getting lost in the One Piece universe. He could end up on a random island and befriend Luffy and his crew. Goku would probably be interested in the different Devil Fruits and might even try to use his powers to help them find the One Piece. His fighting skills would also be a great asset during battles against the likes of the Yonko or the World Government's forces.
Issei would be very likely. Since Issei is the main character in DxD, his curiosity and his own pursuit of power would make him interested in Goku. They could train together and learn from each other's abilities. Also, Rias would interact with Goku. As a powerful devil, she would be intrigued by Goku's power and his different way of approaching battles.
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.
Definitely the potential for epic battles. Picture Goku going up against the Four Emperors all at once. His speed and strength could turn the tides in a battle that the One Piece world has never seen before. Also, his friendship with Luffy could lead to some really heartwarming moments. They both share a love for adventure and fighting strong opponents. And Goku's ability to sense energy could be a game - changer in a world where most rely on observation Haki. It could open up new ways of detecting threats and finding hidden things in the One Piece universe.
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.
Goku would interact with Kale in a way that he wants to explore her powers. He might ask her a lot of questions about how she became so strong. Kale would respond with shyness but also with pride in her abilities. They would start to train together, and Goku would encourage Kale to push her limits. As they train, their relationship would grow, and they would become friends who trust and rely on each other in battles.
Artemis could be intrigued by Goku's energy. She might see it as something similar to the divine energy she's familiar with. So she could start asking him about it. And Goku would try his best to explain about ki, using his limited knowledge of such things. They could end up sharing stories about their respective battles and adventures.
Their interaction could be really interesting. Goku is used to dealing with powerful foes and saving the world in a more action - packed way. Anne Frank is in a situation where she has to be quiet and endure. Goku could teach Anne some self - defense moves in case they are discovered, and Anne could teach Goku about patience and the importance of the written word. She might even write about Goku in her diary, and Goku would be fascinated by this strange new thing called 'writing' that can preserve memories and thoughts.