In a crossover with the Bleach universe, Aizen Sousuke could be a formidable antagonist. His intelligence and powerful abilities would make him a tough enemy for Goku. Aizen could manipulate situations to his advantage, and Goku would have to use his fighting instincts and newfound knowledge of this strange world to defeat him.
The Joker from the Batman universe could also be an interesting antagonist. He could play mind games with Goku, something Goku isn't used to. Goku's straightforward nature would be challenged as he tries to deal with the Joker's unpredictable and often cruel schemes.
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.
Perhaps an ancient evil Saiyan who was banished long ago. This Saiyan would have powers that are unknown to Goku and Vegeta. He could be awakened from a deep slumber and is angry at the current state of the Saiyan race. He sees Goku and Vegeta as traitors for living among the Earthlings and decides to destroy them and take over the universe. His knowledge of ancient Saiyan techniques would give him an edge in battles against Goku and Vegeta.
One idea could be Goku crossing over into a different anime universe, like the Naruto world. He could end up in a fight tournament there and show off his Super Saiyan Blue form. His ki attacks would be a new and strange concept to the Naruto characters.
Maybe a mad scientist - type character who has created a device that can disrupt the fabric of reality. He wants to use it to reshape the universes to his own liking. Goku and Lemon get in his way because they are trying to protect their respective homes. The scientist is constantly evolving his device and creating new threats for Goku and Lemon to overcome.
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.
It could be Frieza. Frieza might see Goku having a family as a weakness and try to take advantage of the situation to finally defeat Goku once and for all.
There could be a power - hungry being who wants to use Goku's mother for their own gain. When Goku tries to meet her, this being tries to stop him because they know that if Goku and his mother reunite, they will be able to oppose their evil plans together.
A powerful demon from another dimension could be the antagonist. This demon, drawn to the energy of the Hyperbolic Time Chamber, tries to take it over. It has the ability to create illusions inside the chamber, making Goku's training even more difficult and dangerous.
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.
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.