Well, they might become friends. Goku, being the kind - hearted boy he is, would probably be curious about Android 18. Maybe they'd start training together, with Goku showing Android 18 some of his martial arts moves. And Android 18 could show Goku some of her advanced technology - like knowledge, if she had any in her android form even as a kid. They could also get into all sorts of adventures, exploring the world around them and getting into harmless little troubles.
Well, at first they might be a bit wary of each other. But as time passed, they'd discover their common interests like the pursuit of strength. Goku's positive attitude would influence Android 18, and she would in turn help Goku with things like problem - solving from her android - smart perspective. As they grew older in the fanfiction, this relationship could become a very deep and important one, with them having each other's backs no matter what.
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.
It could be insanely powerful. Goku already has incredible power levels, and Goku Black is no slouch either. When they fuse, it's like combining two nuclear reactors. Their power could be so great that they could effortlessly destroy planets just by flexing their newfound power. They might be able to reach Super Saiyan forms that are far beyond what we've seen in the official series, with power levels that are basically off the scale.
In fanfiction, if Goku Black fused with Goku, it could potentially create an extremely powerful being. This new entity might have a mix of Goku's pure heart and Goku Black's more sinister motives, which could lead to a complex internal struggle. Their combined power could be off the charts, perhaps strong enough to defeat any foe in the Dragon Ball universe. However, it could also cause chaos as their different personalities clash within the fused form.
There could also be other characters from the Dragon Ball universe making an appearance. For example, Vegeta might show up. He could be curious about Fem Goku and how she compares to Goku in terms of power. Or Bulma could be there, perhaps she's interested in the technology or biology behind Fem Goku's existence.