Goku would probably be really excited to meet Luffy. He'd be curious about Luffy's rubber powers and would want to have a friendly sparring match right away. Luffy, on the other hand, would be in awe of Goku's strength and would see him as a great ally. He'd probably ask Goku to join his crew, but Goku would decline because he has his own friends and family in Dragon Ball world.
Well, it could be about Goku ending up in the One Piece world. Maybe he teams up with Luffy to fight a common enemy. For example, a powerful pirate who has some sort of dragon - like powers similar to the dragons in Dragon Ball. They would combine their unique fighting styles. Goku's energy blasts and Luffy's stretchy attacks would make for an epic showdown against the bad guy.
One possible plot could be that Goku and his friends somehow end up in the One Piece world. They meet Luffy and his crew. Goku might be interested in the Devil Fruits and their powers. Luffy, on the other hand, would be excited to see Goku's super strength and ki abilities. They could team up to fight a common enemy like a powerful pirate who has also acquired some sort of dragon ball - like artifacts. This would lead to an epic adventure with both the Saiyans' and the Straw Hats' unique fighting styles combined.
In a crossover fanfiction, the Dragon Balls could be the key element. Suppose the One Piece characters learn about the Dragon Balls and their ability to grant wishes. Luffy would definitely want to use them to make some crazy wishes, like having an unlimited supply of meat. However, the villains from both universes also get wind of this. Frieza could team up with some of the powerful pirates like Blackbeard. This would force the heroes of both worlds to join forces. Goku and Luffy would lead their respective teams in a battle against this new, powerful alliance, with the fate of both universes at stake.
Well, perhaps the Saiyans in Dragon Ball Z detect the strong energy from the Devil Fruits in One Piece. Vegeta decides to visit the One Piece world to see if he can gain an edge over Goku. Meanwhile, the Straw Hats are dealing with a new threat that seems to be more powerful than any they've faced before. Vegeta initially clashes with the Straw Hats but then realizes they need to work together. They combine their skills, like Goku's Kamehameha and Luffy's Gear Fourth, to take on the big bad.
One possible plot could be that Luffy and his crew are sailing in the Grand Line when they encounter a strange energy. It turns out to be a Dragon Ball. They then have to deal with various villains from both universes trying to get the ball. Maybe Frieza hears about it and teams up with some of the pirates from One Piece. And Goku, sensing the power of the Dragon Ball, also arrives in the One Piece world to help Luffy protect it.
It's likely Goku. His Super Saiyan forms give him incredible power levels that far exceed most of the characters in One Piece. He can move at incredible speeds, has amazing strength, and can fire powerful energy blasts. Even Luffy in his Gear Fifth form would probably have a hard time against Goku at his full power.
Goku and Luffy would be really interesting. Goku is all about pure strength and the pursuit of becoming stronger through training, while Luffy is more about the adventure and his dreams. Their different outlooks could lead to some great conversations and also some funny moments, like when Luffy tries to eat Goku's senzu beans thinking they are some kind of food from his world. Another interesting interaction could be between Vegeta and Zoro. Vegeta is very proud and serious, and Zoro is a swordsman with his own code of honor. They could have some intense sparring sessions and also respect each other's strength.
Goku and Luffy would be an amazing pairing. Goku's pure - hearted love for fighting and his super - strength combined with Luffy's carefree and determined nature would create some really fun interactions. They could both learn from each other's ways of approaching challenges. Luffy could teach Goku about the value of having a loyal crew, while Goku could show Luffy some new fighting techniques.
Zoro and Vegeta would also be a cool pairing. Zoro is all about becoming the greatest swordsman and has a strong sense of pride. Vegeta is a proud Saiyan prince. They could start off as rivals, constantly trying to outdo each other in combat. But as they face tougher enemies together, they would start to respect each other's skills. Vegeta could teach Zoro some energy - based attacks to add to his swordsmanship, and Zoro could show Vegeta the value of discipline in a different way.