One possible adventure is that they explore an uncharted planet. Since they are kids, they would be full of curiosity. On this new planet, they might discover strange creatures and new power - ups. The Saiyan girl could use her intelligence to figure out how to deal with the new environment, while Goku uses his brute strength when they face any threats.
They could have adventures like trying to adapt to the high - gravity environment. Maybe they struggle to learn the Saiyan fighting techniques at first but gradually get better. And they might get involved in some inter - planetary conflicts that the Saiyans are part of.
The Strawhats could have all sorts of adventures. Maybe they team up with Shanks to take on a powerful enemy or discover a hidden treasure.
Perhaps the Saiyan girl has a different fighting style than Goku. When they meet, she could introduce Goku to this new style. They could then blend their styles together to create an even more powerful form of combat. Also, the girl might have a special ability, like being able to sense danger from far away. She could use this ability to protect Goku and they would form a deep friendship based on mutual protection and respect.
Naruto could have all kinds of adventures. He might get involved in a fight against one of the Earth's villains, like Frieza if he somehow showed up. Naruto's Rasengan could be a powerful weapon against such foes. He could also explore different parts of the Earth, like the mountains or the oceans, which are very different from what he's used to in his own world.
They could have an adventure where they help young Naruto in a mission. Maybe Naruto is outnumbered by some enemies and Bolt and Sarada jump in to save the day. Bolt could use his lightning - style jutsu and Sarada her sharingan - enhanced skills.
She could be an original character created by the fanfiction writer. Maybe she has some special abilities like being able to control a unique form of energy that Goku has never seen before.
They could go on a quest to find a powerful dragon ball that has been lost. Goku's sense of adventure and 18's combat skills would be a great combination. Along the way, they would face various enemies. Goku would use his super - fast speed and powerful attacks, while 18 would use her android abilities like energy absorption to deal with the threats.
The Strawhats' encounter with Dadan in fanfic could lead to them getting involved in local mysteries or having to rescue Dadan from some peril. It really depends on the creativity of the author!
They can have flying adventures on the backs of magical creatures.
It could be a new Saiyan character created specifically for the fanfiction. This character could be a strong - willed Saiyan female who has her own set of powers and a unique personality. She might be attracted to Goku because of his strength and his kind heart.