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.
Goku himself is often included in stories with Goku Black. It creates a really interesting dynamic as they are essentially the same being but with different outlooks. The clash between them can be about more than just physical power, but also about their different beliefs and ways of seeing the world.
Another popular character is Beerus. Since he is a powerful god in the Dragon Ball universe, when paired with Godlike Goku, it can lead to some really interesting power dynamics. They could have battles that shake the very fabric of the universe or team up to deal with an emerging evil that threatens the balance of all the realms.
Well, obviously Goku Black himself is very popular. Writers often focus on his character, his power, and his motives.
Future Mai could also be a key character in Goku Black forced fanfiction. Given her relationship with Trunks and her own skills, she could be part of the group that is fighting against Goku Black, or she might even have some unique interactions with him that drive the plot forward.
Well, obviously Black Goku himself is a main character. He is the center of the story in most cases as his actions drive the plot. Goku also features prominently as he is the one who has to oppose Black Goku. Vegeta is often there too, as he is another strong Saiyan and is eager to take on the threat that Black Goku poses.
Sometimes, the Black Joker is paired with other anti - heroes in fanfiction. These pairings can lead to interesting dynamics as they might team up against a common greater evil or have power struggles among themselves.
Severus Snape could also be a character often paired with Bellatrix. They were both on Voldemort's side, but Snape had his own hidden motives. Fanfiction might explore a relationship between them, perhaps a power struggle or a secret alliance that also involves Bellatrix's relationship with Harry in some way.
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.
Naruto can also be paired with Dark Kakashi. Naruto's unwavering belief in the good of people could contrast sharply with Kakashi's darker self. It might lead to situations where Naruto tries to bring back the old, more positive Kakashi, and through their interactions, both characters grow and learn more about themselves.