Well, it could be really epic. Imagine Goku teaming up with Superman. They both have super strength and a strong sense of justice. Goku's martial arts skills combined with Superman's various powers like heat vision could make for some amazing fight scenes against powerful villains.
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.
Some characters might not get along at first. Like Piccolo and Green Lantern. Piccolo is a stoic and often solitary character, while Green Lantern is part of a team. But over time, they could learn to respect each other's power and abilities.
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.
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.
Well, in Dragon Ball Z, power levels are measured in a more straightforward numerical way. Goku at his Super Saiyan levels has an extremely high power level. In the Justice League, Superman is one of the most powerful with his super strength, heat vision etc. In a fanfiction, you could say that Goku's Super Saiyan Blue form might be on par with Superman in terms of raw power, but Superman has other abilities like flight and x - ray vision that Goku doesn't. So it's a complex comparison.
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.
One idea could be a crossover where Goku is accidentally transported to the Justice League's universe. He might initially be seen as a threat due to his power but then they realize he's a good - hearted fighter. They could team up to fight a common enemy like Darkseid who has somehow acquired the Dragon Balls and is using them for his evil plans.
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.
Another likely pair could be Vegeta and Batman. Vegeta is proud and calculating, much like Batman. Batman would be intrigued by Vegeta's Saiyan pride and his combat skills. Vegeta, on the other hand, might respect Batman's strategic mind and his ability to fight without superpowers. Their first interaction could be a tense one, as Vegeta may not immediately trust Batman, but it would be interesting to see how they develop a relationship over time.