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.
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 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.
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.
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 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.
Gohan and Robin would be an interesting pairing. Gohan, with his intelligence and powerful fighting skills, could bond with Robin over their shared smarts and desire to protect their respective worlds. They could exchange knowledge about combat strategies and detective work.
A cool dc marvel crossover fanfiction might involve Batman and Iron Man. Picture Batman being intrigued by Iron Man's high - tech gadgets and the two of them collaborating to create an even more advanced piece of technology to defeat a new breed of super - criminals that are using alien and advanced Earth - based tech.