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.
Another possibility is Frieza. He could have somehow managed to gain new powers and is now back to take revenge on Goku. When he sees the fused Goku, he becomes even more determined to defeat him. Frieza's hatred for Goku combined with his new - found power would make him a formidable antagonist in this fanfiction.
Maybe a new character entirely. Some ancient evil that was awakened by the energy of Goku and Goku Black fusing. This new antagonist could have powers that are completely unknown to the Dragon Ball universe and pose a great threat not only to the fused Goku but also to the entire multiverse.
The plot could involve Goku and Goku Black fusing accidentally. Maybe due to some strange energy anomaly. And then they have to figure out how to control this new form. They might initially be feared by their friends because of Goku Black's reputation, but then they prove that they are on the side of good by using their new powers to protect the Earth from an impending threat that no one else could handle.
Maybe it would be a combined form of some of Naruto and Goku's most powerful enemies. For example, if Frieza and Orochimaru somehow combined their powers and knowledge. They would be a very tricky opponent for the fused Naruto - Goku as they would know the strengths and weaknesses of both heroes.
A new character could be the main antagonist. For example, a powerful alien who has heard of Goku Black's reputation and wants to prove himself by defeating the reincarnated warrior. This alien could have unique powers that are a challenge for Goku Black to overcome, like the ability to nullify ki attacks.
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.
Their fused form might have Goku's spiky hair, but with some of Goku Black's purple - tinted highlights. The body would be muscular like Goku's, but with a more menacing stance similar to Goku Black. The clothing could be a combination of Goku's orange gi and Goku Black's black under - shirt, maybe with some red accents added in for an extra pop of color.
Another option for the main antagonist could be Beerus. As a God of Destruction, he wouldn't be happy with Zamasu and Goku Black causing chaos. Beerus is extremely powerful and has the authority to deal with threats to the universe. He might see their actions as a direct challenge to his power and order. If he gets involved, he could completely change the dynamic of the story, as his power level is on a whole different scale compared to most characters in the Dragon Ball universe.
Maybe an old enemy of Goku's who has somehow managed to return. This enemy has heard about Goku Jr and figures that by attacking him, he can get revenge on Goku. This could be someone like Frieza who has found a new way to be resurrected and is more powerful than ever.
Frieza would be a great antagonist. If Goku goes back in time to a point before Frieza was defeated, Frieza could be an even more menacing force as Goku might not have all the power - ups he had in the original timeline. He'd have to rely on his wits and basic training more.
Maybe an interdimensional warlord who sees Goku as a threat to his conquest plans and Lemon as a valuable asset. He would stop at nothing to capture them and gain their powers. He could have an army of minions from different dimensions, making it really challenging for Goku and Lemon to fight back. This antagonist would be very cunning, constantly devising new strategies to outwit our heroes.
Another possibility is that in the world where Goku Black comes from, there is an enemy who follows Goku Black to Izuku's world. This enemy could be a powerful Saiyan or a being with a dark ki similar to Goku Black. They might see Izuku as a weakness in Goku Black's plan and try to eliminate him first, making them the main antagonist in the fanfiction.