One idea could be that Goku ends up in the world of Aladdin. He might mistake the flying carpet for a new kind of Ki - powered vehicle. And he could use his martial arts skills to help Aladdin fight off Jafar's minions in a really action - packed way.
Another concept is that the Dragon Balls are scattered throughout the various Disney kingdoms. Characters from both universes, like Mickey Mouse and Piccolo, have to team up to find all the Dragon Balls. Along the way, they'd face challenges specific to each kingdom, like dealing with the Evil Queen from Snow White in her castle while searching for a Dragon Ball hidden there.
Perhaps Vegeta could find himself in the world of Frozen. At first, he'd be really frustrated with the cold, but then he'd see Elsa's ice powers and be intrigued. He might challenge her to a battle of powers, thinking he could easily win, but then be surprised by how strong Elsa is. This could lead to an interesting character development where Vegeta learns to respect different kinds of power.
One idea could be a battle between Goku and Superman to determine who is the strongest in the universe. They might start off as rivals but then realize they have more in common than they thought and team up against a greater threat like Darkseid and Frieza combined.
Another interesting concept is to cross them over with a fantasy world. Saiyans could be in a world full of magic and mythical creatures. Picture Vegeta having to deal with dragons that can talk and cast spells. Their Saiyan pride might get in the way at first when they realize that their brute strength isn't always enough in this new world.
A cool concept could be that Vegeta gets transported to Gotham City. He initially clashes with Batman due to their different methods, but they eventually work together to take down a powerful new enemy that is a threat to both their worlds.
Maybe in the fanfiction, Cell could be re - created in a different form with a mix of other characters' DNA. He could have some of Piccolo's regeneration abilities along with Gohan's hidden power. This would lead to a whole new set of battles and power - level dynamics as the Z - fighters try to figure out how to deal with this new Cell.
One idea could be having the characters from RWBY learn the Ki techniques from DBZ. For example, Ruby could combine her speed with the Instant Transmission technique. This would make her even more formidable in battle.
One idea could be Trunks traveling to an alternate universe where the rules of DBZ are slightly different. For example, the Saiyans never lost their planet but have a different culture. Another could be Trunks teaming up with a character from a different anime in the Dragon Ball Z world to fight a new, powerful enemy. Maybe a character from Naruto who has unique jutsu skills that could combine with Trunks' power in an interesting way.
One idea could be to have Goku from DBZ end up in the Bleach universe and learn about soul reapers. He might mistake them for a new type of fighter and try to spar with them. Another could be a fusion of the power systems, like Ichigo getting the ability to go Super Saiyan on top of his existing powers.
Well, a really cool concept could be a villain that threatens both the Naruto and DBZ universes. So, characters from both sides have to team up to defeat this powerful foe. Imagine Vegeta and Sasuke having to work together, despite their initial differences. Also, there could be a story where the Dragon Balls are somehow transported to the Naruto world, and Naruto and his friends try to figure out what they are and how to use them.
One idea could be a crossover between characters from different Disney Infinity franchises in a post - apocalyptic world. For example, Jack Sparrow from Pirates of the Caribbean and Elsa from Frozen teaming up to find a way to restore their shattered universes. Another idea is to have Buzz Lightyear and Stitch on an intergalactic adventure where they have to stop an evil force that is threatening multiple Disney worlds.
One idea could be that Elsa accidentally travels to another Disney world like Aladdin's Agrabah. There, she could use her ice powers in the desert, creating ice sculptures that amaze the locals. Another could be a crossover with The Little Mermaid. Maybe Ariel discovers an iceberg with Elsa trapped inside and helps her get free. They could then explore each other's worlds together.