Loki would also be a difficult adversary. He's very cunning and could use his tricks and illusions to try and outwit Goku. Goku would need to rely on his senses and ki to see through Loki's deceptions.
I think Broly could also be a really tough opponent. In his Legendary Super Saiyan form, he has incredible power and his rage - driven fighting style could give Goku SSJ4 a hard time in the Tournament of Power. Broly doesn't hold back and his raw power could potentially overpower Goku if he's not careful.
Perhaps in the fanfiction, Goku teams up with a human fighter from Record of Ragnarok. They could combine their fighting styles, like Goku teaching the human how to harness ki while the human shows Goku some new hand - to - hand combat techniques.
I think Zeno would be a really tough opponent. Zeno has the power to erase entire universes, and a Goku and Goku Black fusion, despite being extremely powerful, would have a hard time against such a being with god - like powers.
It could be Sasuke. Sasuke is highly intelligent and has a great strategic mind. He would approach shogi with the same intensity as he does his ninja battles. He would be able to analyze Naruto's moves quickly and counter them effectively.
Vegeta could be a great rival. He might see Goku's new position as God of Destruction as another thing Goku has achieved that he hasn't. This could fuel their competitive nature even more.
Madara Uchiha would be a formidable foe. His Sharingan abilities are second to none and his experience in battle is vast. He could easily outwit and overpower the team with his powerful jutsu and strategic mind. Plus, his connection to the Uchiha clan's history gives him an edge in understanding and countering the team's moves.
It might be another new and mysterious trainer in the Kalos league. This trainer could have some rare and never - before - seen Pokémon with unique abilities. But Ash, through his experience and the growth of his Pokémon over time, is able to find the weaknesses of these new Pokémon and defeat this tough opponent. His ability to adapt to different battle situations and his Pokémon's loyalty are what make him victorious.
It could be a female Namekian. Since Namekians are also very strong and have their own unique culture, a female Namekian could catch Goku's eye. They might bond over their mutual respect for life and their experiences with fighting powerful foes.
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.
Madara Uchiha. He is extremely powerful with his Sharingan and Rinnegan abilities. Even if Naruto is a martial arts master, Madara's vast array of jutsu and his strategic mind would make him a formidable foe. He could use his space - time ninjutsu to avoid Naruto's attacks and then counter with powerful attacks of his own.