In a crossover fanfiction, perhaps the Hollows in Bleach start causing chaos in the Dragon Ball Z universe. The Z - fighters are confused by these strange spiritual entities. Ichigo arrives to help deal with the Hollows. He teaches the Z - fighters about the nature of souls and how to detect and fight Hollows. In return, Goku shows Ichigo new ways to increase his power level. They form an alliance and together they train to become even stronger so they can stop a powerful Hollow - like being that is emerging as a major threat.
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.
One idea could be that the Soul Reapers from Bleach team up with the Z - Fighters from Dragon Ball Z to fight a common powerful enemy. For example, a new type of demon that has the ability to absorb spiritual energy like in Bleach and also has the strength and fighting skills similar to the villains in Dragon Ball Z.
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 cool plot might involve a rift in space - time that pulls the Dragon Ball Z world and the DC Universe together. Vegeta could be at odds with Wonder Woman at first, seeing her as a rival in terms of strength and combat skills. The Justice League and the Z Fighters then discover that an ancient evil force from both of their universes' pasts has been awakened. They must learn to work together, with Gohan and Robin sharing their intelligence to figure out a plan. The Saiyans' transformation abilities and the Justice League's various powers like the Flash's speed would be crucial in the final battle against this evil.
Perhaps a magic spell in the Fairy Tail world goes wrong and creates a portal that sucks in some of the main characters from Dragon Ball Z. Vegeta, for example, would initially be angry about being in this strange new place but would soon realize that there are powerful foes and allies here. The Fairy Tail guild could hold a tournament to see who is the strongest among them and the Saiyans join in. This would lead to epic battles and also some comical moments as the two groups with very different cultures try to understand each other.
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 somehow gets his hands on the Dragon Balls. He wishes for the power to snap away half of all life in the universe even more easily. The Z - fighters sense this great imbalance and travel to the Marvel universe to stop him. Goku, being the most powerful Z - fighter, tries to fight Thanos but quickly realizes his strength is different from what he's used to. Vegeta might have to team up with Iron Man to come up with a technological way to counter Thanos' Infinity Stones while Goku trains to reach a new level of Super Saiyan to take on Thanos directly.
Perhaps the Dragon Balls get scattered across the Yu - Gi - Oh world. Yugi and his friends, along with Goku and the Z - fighters, have to search for them. But there are evil forces in the Yu - Gi - Oh world that want to use the Dragon Balls for their own gain. So, they have to battle these foes while also dealing with the various challenges and mysteries of the Yu - Gi - Oh universe, like the different types of card decks and the power of the Shadow Realm.
One idea could be a crossover with the Naruto universe. Goku and Naruto could team up to fight a common enemy. Maybe they face an evil villain who has the power - stealing abilities like Cell and the ability to use powerful ninja jutsus. Another plot could be a crossover with One Piece, where Luffy and Goku have a friendly competition about who can eat more food and then end up having to work together to defeat a new threat at sea that has the strength of Saiyans and the deviousness of pirates.
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.
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.