Obviously, Goku Black himself is the key character as the story revolves around his transformation. Goku would also be important as he could be the one who shows Goku Black the path of good, either directly or indirectly.
Vegeta could play a crucial role. He might be initially skeptical of Goku Black's change but could later become an ally. And Whis, with his wisdom, could help guide Goku Black on his journey to becoming good. Also, Bulma could be involved, perhaps using her intelligence to create something that aids in Goku Black's transformation.
One possible plot could be that Goku Black starts to have doubts about his evil actions after experiencing some self - reflection. Maybe he witnesses an act of great kindness that makes him question his motives.
It could lead to a really interesting plot twist. Maybe he realizes the error of his ways and decides to use his power for good.
Vegeta would be a great choice. Their rivalry in the original Dragon Ball series could be taken to a whole new level in a crossover setting. Another good character could be Trunks. Since Goku Black has a connection to Trunks in the main story, it would be interesting to see how they interact in a different scenario.
Bulma could be a catalyst. She's very intelligent and might come up with some sort of device or plan that could change Goku Black's perspective. Maybe she creates a machine that can influence his mind just enough to make him start to question his evil ways.
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.
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.
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.
Well, obviously Goku Black himself is very popular. Writers often focus on his character, his power, and his motives.
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.
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.