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.
An artificial being created by a mad scientist. The scientist, having studied the Hyperbolic Time Chamber, creates this being with the sole purpose of defeating Goku. The being has the ability to adapt to any attack and can also manipulate the time - like effects inside the chamber to its advantage, making it a formidable foe for Goku.
One idea could be Goku discovers a new power within the Hyperbolic Time Chamber. For example, he might accidentally tap into a form of energy that allows him to manipulate time outside the chamber in a very limited way. This new power could be a key to defeating an upcoming, extremely powerful enemy.
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.
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 a rogue Saiyan from an alternate timeline. This Saiyan could be jealous of Goku's status as the strongest Saiyan in his own timeline. So, they use time travel to come to Goku's timeline and try to defeat him and take over as the dominant Saiyan.
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.
For Naruto, it could accelerate his growth not just in terms of power but also in maturity. Spending a long time alone in the chamber, he might have deep self - reflections and come out with a better understanding of his purpose and responsibilities.
One idea could be Naruto using the hyperbolic time chamber to train intensively for a new and extremely powerful jutsu. He could emerge with a whole new set of skills that surprise his friends and foes alike.
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 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.
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.