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 third idea is to have a crossover within different time periods in the DBZ universe itself. For instance, take Saiyans from the pre - Frieza era and have them cross over to the time of the Tournament of Power. They would be shocked by the new level of power and the different races that exist. It could lead to some interesting battles and alliances as they try to adapt to this new 'future' of their own universe.
Well, you could do a time - travel based crossover fanfiction. Have a legendary super saiyan like Vegeta travel back in time to a different era in the Dragon Ball Z universe. He could meet his younger self and together they could face a new threat that was previously unknown. Or, a crossover with a video game world. Picture the legendary super saiyan Trunks entering the world of Final Fantasy and having to go on a quest there.
A great concept for a Super Saiyan crossover fanfiction would be to cross it with the Harry Potter world. Imagine a Super Saiyan wizard or witch. They could use magic spells while in their Super Saiyan form. Or perhaps a crossover with the God of War series, where the Super Saiyans fight alongside Kratos against the gods of that universe.
For a 'godlike saiyan crossover fanfiction', a crossover with the Marvel Universe would be exciting. Picture a godlike Saiyan arriving on Earth during the events of 'Avengers: Endgame'. They could help the Avengers in their fight against Thanos. Their super - strength and energy - blasts could turn the tide of the battle. Or perhaps a crossover with the 'Lord of the Rings' world. The Saiyan could be a powerful ally in the fight against Sauron, using their speed and power to take on the orcs and other dark creatures.
Another concept could be a Saiyan - Naruto hybrid. A character who has both the natural talent for ninjutsu like Naruto and the raw power of a Saiyan. This character could struggle with balancing the two different power sources and cultures. They might face discrimination from both the Saiyan and ninja communities until they prove themselves as a unique and powerful force.
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.
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.
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.