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.
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.
Another possible main character could be the villain that forces the two Gokus to meet. This villain could be a new, super - powerful threat that the two Gokus need to deal with. Their meeting becomes a crucial part of the story to defeat this powerful foe.
In some 'Goku meets Goku fanfiction', they could team up against a super powerful enemy that neither could defeat alone. They would combine their signature moves like the Kamehameha for an ultimate attack.
The plot could involve Goku and Goku Black fusing accidentally. Maybe due to some strange energy anomaly. And then they have to figure out how to control this new form. They might initially be feared by their friends because of Goku Black's reputation, but then they prove that they are on the side of good by using their new powers to protect the Earth from an impending threat that no one else could handle.
Well, perhaps in this fanfiction, Goku Black makes a fatal mistake. He could underestimate Goku's ability to adapt during battle. Goku, being the experienced fighter he is, notices this weakness and uses it against Goku Black. He might use a combination of his signature moves like the Kamehameha in a new and unexpected way that catches Goku Black off - guard, leading to his defeat.
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.
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.