In the manga, Goku managed to kill Zamasu by combining his strength with Vegeta and performing a joint attack. They also exploited Zamasu's weaknesses to gain the upper hand.
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.
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.
A common theme is power - seeking. They are often depicted as constantly striving for more power and ruling over the universe. Another theme is their unique bond, whether it's a friendship or a more sinister partnership.
Well, if they were brothers in a fanfic, it could change how they interact with the other characters. Maybe they'd have a deeper understanding and fewer power struggles between them. Also, their family background and upbringing could be explored in new and interesting ways.
It's a bit hard to say for sure. Both Vegeta and Goku have their moments of strength. Goku has some unique powers and techniques that give him an edge at times, but Vegeta is no slouch either and constantly works to catch up and surpass Goku.