Well, Vegeta would be a likely character. He might be initially suspicious of the female Goku. Maybe he thinks she's a threat or just can't believe there's another Goku - like being. He could be there to add some comical moments as he tries to understand the situation.
A great fanfiction could be Whis falling in love with a new character. This new character might be a powerful being from a different galaxy. Their relationship would bring a whole new dynamic to the story. Whis would have to balance his duties as an attendant to Beerus with his new - found feelings. Also, there could be a story where Whis creates a new form of training for Vegeta that helps him finally surpass Goku in power, which would be a huge twist in the Dragon Ball Super narrative.
In general, it's a bit of a toss-up. The strength of Manga Goku and Anime Goku can vary depending on the specific arcs and battles. Sometimes, it might seem like one is stronger in a particular situation, but that could change in another.
Obviously, Whis is a main character in most of these fanfictions. He is often the center of the story, whether it's about his powers, his relationships, or his role in the Dragon Ball Super universe. Goku also frequently appears in these stories. Since Goku is always eager to learn and become stronger, Whis is often his mentor in fanfictions, so their interactions are a common theme.
Well, in fanfiction, if Goku Black fuses with Goku, it could result in a super powerful being. Their combined powers might create a new form with an extremely high power level. This new entity could have a mix of Goku Black's dark and devious nature with Goku's pure-hearted heroism, leading to an internal struggle within the fused form.
Erza would also be a good pairing. She is a very disciplined and strong - willed character. Goku could learn from her discipline, and she could learn from Goku's carefree yet determined attitude. They could fight side by side in battles and protect each other.
If Goku Black fuses with Goku in fanfiction, the story would progress in a complex way. The fused form might go on a journey to understand itself. Since it has two very different personalities combined, it would face a lot of internal conflict. This could be used to explore themes like identity and the struggle between good and evil. Externally, the fused Goku could attract new enemies who see it as a threat to their power or plans.