In such a fanfiction, it could lead to some interesting plot twists and character developments. Maybe Goku's identity as Kakarot would be explored in a deeper and more complex way.
With his son Gohan, the relationship could be strained. Gohan has always looked up to Goku as a hero. But as Kakarot, Goku might push Gohan to be more like a traditional Saiyan, which could be too much for Gohan. Also, Vegeta's view of Goku would change. Vegeta has always seen Goku as a rival but also respects him. As Kakarot, Vegeta might see him more as a threat to the Saiyan way of life or as a potential ally in a more Saiyan - centered agenda.
One possibility is that Goku starts to remember more of his Saiyan past as Kakarot. Maybe he begins to have flashes of his original mission on Earth, which was to destroy it. This could create an internal conflict within him as he has grown to love Earth and its people. He might struggle with his identity, trying to reconcile the ruthless Saiyan warrior Kakarot was supposed to be with the kind - hearted Goku he has become.
Zamasu is often paired with Goku Black. Since they are closely related in the original story, fanfiction writers like to explore their relationship further. It could be about their shared goals or how they interact on a more personal level.
One interesting fanfiction could be about Goku and Black Goku teaming up against an even greater threat from another universe. In this story, they might initially be at odds but gradually learn to work together, using their unique abilities to save the multiverse.
In fanfiction about Goku and Goku Black, a common concept is the alteration of their power levels. Writers may give Goku new forms or boost Goku Black's abilities in unexpected ways. There can be stories centered around Goku Black's adaptation to the main timeline's culture and society. For example, he might have to deal with the everyday life in addition to his evil plans. And there are fanfictions that focus on the psychological impact of their battles on each other, like how Goku might be affected by constantly facing an evil version of himself.
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.
It could be insanely powerful. Goku already has incredible power levels, and Goku Black is no slouch either. When they fuse, it's like combining two nuclear reactors. Their power could be so great that they could effortlessly destroy planets just by flexing their newfound power. They might be able to reach Super Saiyan forms that are far beyond what we've seen in the official series, with power levels that are basically off the scale.
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.