There are multiple factors. Firstly, the fanfiction allows for exploration of new storylines that the original shows might not cover. For instance, a fanfiction could explore what would happen if Goku Black was brought to the RWBY world and had to team up with the main characters to fight a common enemy. Secondly, the fanfiction can play with the different aesthetics of the two universes. The bright and colorful world of RWBY contrasted with the more intense and action - packed world of Goku Black can create a visually and thematically interesting blend. Moreover, the fanfiction gives fans the freedom to reshape and reimagine the characters in new and exciting ways.
The transformation aspect could also be explored. Goku Black has his different forms in the Dragon Ball series. In RWBY fanfiction, these forms could be affected by the Aura or magic in the RWBY world. Maybe his Super Saiyan forms could be enhanced or restricted by the unique rules of RWBY.
One idea could be a crossover where Goku somehow ends up in the RWBY universe. He could train the RWBY characters, sharing his martial arts knowledge and helping them improve their combat skills. For example, he could teach Ruby how to better control her speed and use her scythe more effectively in combat.
One interesting fanfiction could be about Goku and Black Goku teaming up against an even greater threat from another universe. In this story, they might initially be at odds but gradually learn to work together, using their unique abilities to save the multiverse.
In fanfiction about Goku and Goku Black, a common concept is the alteration of their power levels. Writers may give Goku new forms or boost Goku Black's abilities in unexpected ways. There can be stories centered around Goku Black's adaptation to the main timeline's culture and society. For example, he might have to deal with the everyday life in addition to his evil plans. And there are fanfictions that focus on the psychological impact of their battles on each other, like how Goku might be affected by constantly facing an evil version of himself.
Another factor is the plot. It needs to be engaging. Maybe it could explore Goku Black's origin in a new way or have him face off against the Z - fighters in creative battles. A fanfiction that has a good balance between action, character development, and plot twists can be really great. For instance, if it shows how Goku Black's actions affect the different planets in the Dragon Ball Z cosmos, that would add depth to the story.
One possible storyline could be Goku Black's arrival in the RWBY universe causing chaos as his power level far exceeds that of most characters initially. He might target the powerful relics in the RWBY world, believing they could further his own dark ambitions.
One interesting concept could be the power dynamics. Since Black Goku is a powerful and often malevolent version of Goku, exploring how their powers interact in a fanfiction can be exciting. For example, they might have an epic battle where their similar yet different fighting styles clash.
Goku Black could also be a part of some sort of experiment gone wrong in the RWBY world. Maybe some scientist was trying to combine different types of energy and accidentally brought Goku Black into their realm. This would lead to a lot of chaos as he starts causing trouble and the RWBY teams have to stop him.
You can start by looking on fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. AO3 has a great tagging system, so you can search for 'Goku Black' and 'RWBY' tags to find relevant fanfictions. FanFiction.net also has a large collection, but the search might be a bit trickier. Another option is to check out Reddit. There are subreddits dedicated to both Goku Black and RWBY where fans might share or recommend fanfictions.
One idea could be that Goku and Goku Black are forced to team up against an even greater threat from another universe. They start to understand each other's fighting styles during the battle and gradually develop a grudging respect for one another.