It could lead to a completely different story. Maybe Goku wouldn't have the same powers or personality traits that we know and love.
Well, he could change a lot of things. Maybe he'd prevent some bad events from happening in the past. For example, he could stop Frieza from destroying Planet Vegeta earlier. It would be really cool to see how his super saiyan power affects the past timeline.
The female Saiyan might be stronger than Goku at first. This could lead to an interesting dynamic where Goku has to find new ways to train and improve. He could learn some new Saiyan techniques from her. For example, she might know a special form of ki - blasts that are unique to her family.
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.
The story would be quite different. Goku might have more freedom to pursue his battles without having to worry about Chichi's concerns. It could also affect the relationships with his kids and the way he interacts with the other characters.
Well, he might change some crucial events. For example, he could prevent some villains from ever rising to power. Maybe he'd stop Frieza from destroying Planet Vegeta earlier, which would change the whole Saiyan history and his own life. He could also teach his past self some new techniques, making the Saiyans even stronger in that timeline.
A Goku and Goku Black fusion in fanfiction could be really exciting. The fusion would inherit Goku's love for fighting and getting stronger, but also Goku Black's ambition. Their combined ki could be a new type of energy that no one has ever seen before. This new fused form might also have new transformations or abilities. For example, it could combine Goku's Super Saiyan forms with Goku Black's Rose form in unexpected ways. It could change the power balance in the Dragon Ball world and open up all sorts of new storylines, like teaming up with other characters to take on new threats or even having internal struggles between the two personalities within the fusion.
In fanfiction, if Goku Black fused with Goku, it could potentially create an extremely powerful being. This new entity might have a mix of Goku's pure heart and Goku Black's more sinister motives, which could lead to a complex internal struggle. Their combined power could be off the charts, perhaps strong enough to defeat any foe in the Dragon Ball universe. However, it could also cause chaos as their different personalities clash within the fused form.
Well, in fanfiction, if Goku Black fuses with Goku, it could result in a super powerful being. Their combined powers might create a new form with an extremely high power level. This new entity could have a mix of Goku Black's dark and devious nature with Goku's pure-hearted heroism, leading to an internal struggle within the fused form.
If Goku married a Saiyan in a fanfiction, it might bring new challenges and adventures. Maybe there'd be family feuds or combined efforts to protect their kind. It really depends on how the author envisions their relationship and the subsequent story arcs.
One trait is his love for combat. Goku would constantly seek out strong opponents to test his skills. Another is his Saiyan pride. He might not back down from a challenge even if it seems impossible. Also, his Saiyan appetite. He would probably eat huge amounts of food to fuel his Saiyan - level energy.