Nora could also be a likely candidate. She's very energetic and not one to be intimidated easily. When she sees Goku's amazing power, she would probably be excited and want to team up with him right away. She would be constantly asking Goku about his world and his powers, and they would bond over their love for combat and having a good time.
One idea could be that Goku accidentally travels to the RWBY universe during a sparring session that goes wrong with Whis. He could be mistaken for a powerful Grimm at first, causing chaos. Then, he teams up with Team RWBY to fight a new super - strong Grimm that threatens all of Remnant.
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.
Issei would be very likely. Since Issei is the main character in DxD, his curiosity and his own pursuit of power would make him interested in Goku. They could train together and learn from each other's abilities. Also, Rias would interact with Goku. As a powerful devil, she would be intrigued by Goku's power and his different way of approaching battles.
Ruby. She's impulsive and has a love for food, especially cookies. It's easy to imagine her getting excited about a food fight and accidentally starting one by playfully throwing a cookie at Weiss or someone else, which then escalates into a full - on food fight.
Reinhardt and Yang would be quite a pair. Reinhardt is the fearless crusader with his rocket - powered hammer and shield, while Yang is a brawler with her gauntlets. They could have some epic battles side by side, and Yang could learn about honor and chivalry from Reinhardt, while Reinhardt could be impressed by Yang's tenacity and determination in the face of danger.
Well, in this kind of fanfic, it's not so simple to decide who is stronger. Goku's power levels keep increasing exponentially as he reaches new super saiyan forms. He can destroy planets with his power. Naruto, though, has the ability to harness the power of the nine - tailed fox which is also extremely powerful. Moreover, Naruto has his own unique chakra - based attacks. If they were to fight, Goku might start off with an advantage due to his speed and power. But Naruto is a very quick learner and could adapt during the fight. Also, Naruto has his friends and allies that could potentially tip the balance in his favor in a more complex battle scenario.
Sasuke could also oppose Black Goku. With his Sharingan and Rinnegan, Sasuke has a wide range of powerful abilities. He is also very intelligent and strategic. Sasuke would likely see Black Goku as a threat to the balance of power in the Naruto universe and would step in to fight him, especially if it involved protecting his friends or the ninja world in general.
Yang and Tucker could also work well together. Yang is strong and confident, while Tucker has his own brand of bravado. They could get into all sorts of adventures and battles, with Yang using her brawler style and Tucker with his sword and unique abilities. Maybe they'd compete for who can take down the most enemies in a fight.
Well, it could be a team effort actually. Ruby could handle the more action - oriented flying parts, Weiss could manage the navigation and communication systems, Blake could keep an eye out for any threats using her stealthy skills, and Yang could be in charge of the power systems and any emergency repairs. This way, all of their skills are combined to effectively pilot the space shuttle in the RWBY fanfic.
The members of Team JNPR could also be likely survivors. Jaune has grown a lot in his skills. His leadership and swordsmanship could be useful. Pyrrha, if she were still alive in this fanfic scenario, would be a great asset with her combat abilities. Ren's calm and collected nature, along with his stealth and combat skills, would help him survive. And Nora, as mentioned before, would be a great addition to the group of survivors. They could work together with Team RWBY to face the zombie apocalypse.