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.
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 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.
Goku Black's super strength could be relative to the Grimm in RWBY. Since the Grimm are often very strong and durable, Goku Black's strength might need to be adjusted slightly so that he doesn't completely overpower them all at once. His strength could be on a level where he can take on multiple high - level Grimm at the same time, but still face challenges from the strongest ones.
Well, Goku Black could be a powerful antagonist in the RWBY fanfic. His combat skills and powers from the Dragon Ball universe would make him a formidable foe. He could have his own motives, like seeking power in the RWBY world or trying to impose his own sense of order. For example, his ability to use ki blasts could be a new and terrifying threat to the Huntsmen and Huntresses in RWBY.
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.
A Goku and Goku Black fusion in fanfiction could be really exciting. The fusion would inherit Goku's love for fighting and getting stronger, but also Goku Black's ambition. Their combined ki could be a new type of energy that no one has ever seen before. This new fused form might also have new transformations or abilities. For example, it could combine Goku's Super Saiyan forms with Goku Black's Rose form in unexpected ways. It could change the power balance in the Dragon Ball world and open up all sorts of new storylines, like teaming up with other characters to take on new threats or even having internal struggles between the two personalities within the fusion.