One idea could be that Goku and his friends are on a quest to find the rarest Pokemon in the Dragon Ball Z world. Maybe the Dragon Balls have the power to summon legendary Pokemon. For example, the Saiyans could use their fighting skills to train Pokemon for battle. Another idea is that the villains from Dragon Ball Z, like Frieza, try to capture powerful Pokemon to use in their evil plans.
Another concept could be that Sonic and his friends end up in the Dragon Ball Z world due to some sort of interdimensional rift. They have to adapt to the new world with its different power levels and fighting styles. Sonic could teach some of the Z - fighters his unique spin - dash and homing attack moves, while Goku could show Sonic how to harness and control ki energy.
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 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.
One idea could be to have Goku and his friends end up in the RWBY world. They could team up with Team RWBY to fight a common enemy. For example, the Saiyans' power levels would be a huge asset against the Grimm. Another idea is to have Ruby Rose gain the ability to use Ki like in Dragon Ball Z. This would change the way she fights and add a new dimension to her character.
Well, a plot could be Android 21 somehow getting her hands on the Namekian Dragon Balls. Since she has that scientific mind, she might try to figure out how to modify the wishes. In the process, she could draw the attention of Frieza who wants the Dragon Balls for his own evil purposes. Then the Z - fighters would have to step in to protect Android 21 and the Dragon Balls. Another plot idea is that Android 21 is from an alternate universe where she was created by the good guys. When she crosses over to the main Dragon Ball Z universe, she has to adjust to the different power levels and politics among the characters. And she could end up being a key player in the fight against Buu, perhaps using her unique abilities to weaken Buu in some way.
Well, a cool plot might be that the Sage of Six Paths in Naruto has a connection to the Kais in Dragon Ball Z. This connection leads to a situation where Naruto and Goku have to work together to stop an ancient evil that threatens both universes. For example, the evil could be a powerful demon that feeds on both chakra and ki. They would have to combine their powers and the powers of their friends to stand a chance. Naruto could teach Goku some of his shadow clone jutsu, and Goku could show Naruto how to achieve Super Saiyan forms.
One idea could be Gohan crossing over into the Naruto universe. He could team up with Naruto and use his Saiyan powers to fight against the Akatsuki. Another plot could be Gohan in the Harry Potter world, using his ki to defend Hogwarts from Voldemort. Or perhaps Gohan in the DC universe, fighting alongside Superman against a common enemy.
Well, a cool plot could be that the Time Patrol discovers a rift in time that leads to a parallel Dragon Ball Z universe where the Saiyans never came to Earth. This would change the entire dynamic of the story. The Saiyans on Earth, like Goku and Vegeta, would have to figure out how to deal with this new universe's threats without their Saiyan heritage. Or perhaps the Time Patrol has to stop a powerful time - traveling wizard from rewriting the history of the Dragon Ball Z universe to make himself the supreme ruler. He could be using his magic to manipulate the Z - fighters' memories and abilities. Also, a story where the Time Patrol recruits a young, untrained character from the DBZ world to be part of their team and trains them to become a powerful time - warrior could be really exciting.
In some fanfictions, Goku could team up with Ash. Goku's super strength and ability to fly would be a great asset. He could help Ash in battles against tough opponents like Team Rocket when they use more powerful Pokemon or machinery. Also, Goku might be intrigued by the concept of Pokemon evolution, which is somewhat similar to Saiyan power-ups.
Well, for 'Ben 10 Dragon Ball Z fanfiction', a cool concept could be that Ben gets caught in a time - space rift that sends him to the Dragon Ball Z universe right when a new and powerful enemy is emerging. Ben has to quickly figure out how to use his Omnitrix in this new world while also trying to get back home. And perhaps some of the Dragon Ball characters are intrigued by the Omnitrix and want to study it, leading to some funny and interesting interactions.