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.
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.
Definitely characters from other shonen anime. For instance, Luffy from One Piece. His rubbery abilities and carefree attitude would contrast well with the serious and powerful super saiyans. Another good character could be Ichigo from Bleach. His sword - fighting skills and unique power could make for an interesting interaction with the super saiyans' ki - based attacks.
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.
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.
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.
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.
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 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.