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 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.
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.
One possible plot could be that Thanos has somehow obtained the Dragon Balls. The Avengers team up with Goku and his friends to get the Dragon Balls back and stop Thanos from using them for his evil plans. They have to battle Thanos' forces while also dealing with the different power levels and fighting styles between the two universes.
One possible plot could be that Naruto and his friends are facing a powerful new enemy in their world. However, this enemy has some sort of connection to the Dragon Balls. Goku and Vegeta sense the strange energy from Naruto's world and travel there using Instant Transmission. They then team up with Naruto and the others. Naruto is amazed by Goku's strength and ki abilities, while Goku is intrigued by Naruto's chakra and ninja skills. Together, they combine their powers to defeat the common foe.
The new storylines that can be created. For example, a new villain could emerge that requires the combined efforts of both the One Piece and Dragon Ball Z crews to defeat. This could involve battles in different settings from both universes, like fighting on the Grand Line and also in the Hyperbolic Time Chamber. It gives a whole new perspective to the existing story arcs of both series.
It could be wild! Imagine the Straw Hat Pirates teaming up with Goku and his friends to take on epic battles and adventures.
A cool plot could be a tournament - style crossover. A powerful being from another dimension decides to host a tournament where fighters from Dragon Ball Z and Devilman are invited. Characters like Vegeta and Devilman's friend Akira Fudo would compete. There could be unexpected alliances and rivalries. For example, Piccolo might initially see Devilman as a threat but later realize they have a common goal of protecting their respective worlds. And in the end, they all have to team up against the true evil that the tournament organizer has in store.
A plot might involve the Dragon Balls being scattered across the Bleach universe. Ichigo hears rumors about these powerful orbs and starts looking for them. Meanwhile, Goku and his friends also end up in the Bleach world while searching for the Dragon Balls. They meet the Soul Reapers and at first there's some tension. But as they learn more about each other's goals, they work together. The Soul Reapers are fascinated by Goku's power and training methods, and Goku is intrigued by the concept of souls and the afterlife in the Bleach world.
The most exciting part is seeing the different power sets interact. Sonic's speed and agility against Goku's super strength and ki blasts make for some really cool battle scenes.
The most exciting part is seeing the different power sets interact. Like Thor's lightning powers against Vegeta's energy blasts. It would be a show of raw power and strategy.