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.
The plot could involve Goku and Goku Black fusing accidentally. Maybe due to some strange energy anomaly. And then they have to figure out how to control this new form. They might initially be feared by their friends because of Goku Black's reputation, but then they prove that they are on the side of good by using their new powers to protect the Earth from an impending threat that no one else could handle.
Well, perhaps in this fanfiction, Goku Black makes a fatal mistake. He could underestimate Goku's ability to adapt during battle. Goku, being the experienced fighter he is, notices this weakness and uses it against Goku Black. He might use a combination of his signature moves like the Kamehameha in a new and unexpected way that catches Goku Black off - guard, leading to his defeat.
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.
The length of the DBFZ story can be subjective. It includes multiple arcs and character developments, which can make it feel quite lengthy for some players, while others might breeze through it.
Character development is also crucial. In many such fanfictions, Female Goku Black may start as a villainous or at least antagonistic character. But as the story unfolds, she might change due to her interactions with Goku. This transformation can be really interesting to follow. For example, she could start to question her own motives or even develop feelings for Goku, which is a big shift from her initial stance.
It could be Zamasu. Since Goku Black was Zamasu in Goku's body in the original story, perhaps a more powerful and enraged Zamasu could be the main antagonist. He might be jealous of the new fused form's power and seek to destroy it.