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.
Maybe there's a rift in the multiverse that merges parts of the Dragon Ball Z world and the DC Universe. The Z Fighters and the Justice League initially think they are enemies due to misunderstandings. For instance, when Goku first meets Batman, Batman might mistake Goku's carefree attitude for recklessness. But as they gradually figure out the situation, they team up to close the rift and fight off threats that are emerging from the merged worlds.
I think Gohan and Martian Manhunter would make for an interesting duo. Gohan is a brilliant and powerful character who has a lot of potential. Martian Manhunter, with his telepathic abilities and shape - shifting, could teach Gohan new ways to use his powers. And Gohan could show Martian Manhunter the value of pure physical strength. Also, Trunks and Flash would be a cool pair. Trunks' time - traveling and sword - fighting skills combined with Flash's super - speed would create some really exciting action scenes. They could race each other or team up to outrun a powerful enemy.
One idea could be a crossover where Goku meets Superman. They could have a friendly sparring match to test each other's strength. Maybe they start off misunderstanding each other's motives but then realize they are both protectors of their respective universes.
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.
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.
Well, the plot could involve the Z - fighters teaming up with the Justice League to fight a common, extremely powerful enemy. Maybe Darkseid teams up with Frieza, and their combined forces are too much for either group alone. So, Goku and Superman start training together to learn from each other's strengths, like Goku learning about Superman's heat vision and Superman learning Goku's Instant Transmission.