In the fanfic, perhaps Goku's transformation into a girl is the result of a magical spell. As a girl, she might start to see the world from a different perspective. She could be more in touch with her emotions and this could either enhance or hinder her fighting abilities. For instance, she might be more empathetic towards her enemies, which could lead to some interesting moral dilemmas during fights. Her training regime might also change as she adapts to her new body.
Well, one possible plot could be that Goku, as a girl, has to face different challenges in the martial arts world. Maybe the other fighters underestimate her at first, but she quickly proves them wrong with her incredible strength and skills. She might also have to deal with new emotions and how others perceive her femininity in the context of being a powerful fighter.
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.
Vegeta could be a great rival. He might see Goku's new position as God of Destruction as another thing Goku has achieved that he hasn't. This could fuel their competitive nature even more.
Goku does reach a state of immense power in the manga that rivals that of a God of Destruction. However, the specific title and all its implications might not be straightforwardly assigned to him. It's a complex development within the story.
The character development could be interesting. Goku, now a girl, would have to adapt to different social expectations. She might become more empathetic as she experiences some of the things that female characters in the Dragon Ball universe face. For example, she could be more in tune with the emotional needs of her friends and family.
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 would be a great addition. His rivalry with Goku and his pride as a Saiyan would add an interesting dynamic. He might be initially jealous that Goku is getting to fight and train with the God of Destruction. But then he would also want to prove himself and join in the action. Whis could also be included. His knowledge and calm demeanor could help in situations where the power of Goku and the God of Destruction gets out of hand.