Who could be the antagonist in a Goku Hyperbolic Time Chamber fanfiction?

It could be a rogue Saiyan from another part of the universe who somehow manages to enter the hyperbolic time chamber. This Saiyan might have a grudge against Gohan's family or be on a quest for power and sees Gohan as an obstacle.

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.

A new character created specifically for the fanfiction could be the main character. This character could be an underdog, not as strong as the well - known fighters. But in the hyperbolic time chamber, they discover their hidden potential and strive to prove themselves against the more established heroes.

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 an ancient evil ninja who was sealed away long ago and somehow gets released in the time chamber. Naruto has to face this powerful foe while also dealing with the time - warped environment of the chamber.

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.

One idea could be a character going into the chamber not to train for fighting but to study ancient knowledge that can only be accessed within the unique time - space of the chamber.

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.