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.
I think Zeno would be a really tough opponent. Zeno has the power to erase entire universes, and a Goku and Goku Black fusion, despite being extremely powerful, would have a hard time against such a being with god - like powers.
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.
Sasuke could also oppose Black Goku. With his Sharingan and Rinnegan, Sasuke has a wide range of powerful abilities. He is also very intelligent and strategic. Sasuke would likely see Black Goku as a threat to the balance of power in the Naruto universe and would step in to fight him, especially if it involved protecting his friends or the ninja world in general.
Hit would also make for a fascinating pairing. Hit is a skilled assassin with time - manipulation abilities. He could be a great foil to Goku Black. Maybe Goku Black tries to outwit Hit in his time - stop techniques, or they could form a temporary alliance based on their mutual desire to eliminate certain opponents in the tournament. There would be a lot of cat - and - mouse action between them.
The main characters are Goku Black (Zamasu in Goku's body), Goku, Vegeta, and Zamasu in his original form. Goku and Vegeta are the ones trying to stop Goku Black's evil plans.
Well, Goku Black could be the more dominant character. His dark nature gives him a different kind of power. He doesn't have the same moral constraints as Goku. This could allow him to make more ruthless and effective decisions in battle. He might be able to outmaneuver Goku if he plays his cards right.
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.