A possible plot could be that Goku enters the Hyperbolic Time Chamber to train for a battle against a foe who has the ability to nullify ki attacks. Inside the chamber, he has to relearn all his combat techniques and develop a new style of fighting that doesn't rely on ki. He discovers ancient martial arts forms inscribed on the walls of the chamber and uses them to create a unique fighting style.
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 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.
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.
Perhaps in the fanfiction, Gohan invites some of his friends or family to the hyperbolic time chamber for a special training session. They could all learn from each other and develop new team - based techniques. This would also add an element of camaraderie and character development as they face the challenges of the time - altered environment together.
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 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.
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.
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.
Well, the plot could be that Goku and Android 18 are sent back in time by a powerful enemy's time - warping device. They end up in a time when the Saiyan race was still thriving on Planet Vegeta. Goku, being a Saiyan himself, is conflicted about interfering with the past. Android 18, on the other hand, is more interested in finding a way back. But as they stay there, they start to realize that there are things they can do to make the future better, like preventing certain evil Saiyans from doing terrible things that would later have far - reaching consequences for the universe.
One possible plot could be that Goku somehow gets transported to the Bleach universe. He might initially be confused by the concept of Hollows and Soul Reapers. But then, with his power, he could start fighting Hollows, thinking they are some sort of new villains. His ki-based attacks would be a new and powerful force in the Bleach world, and he could end up teaming up with Ichigo to fight a common, extremely powerful enemy, like a super - powerful Hollow that even the Soul Reapers are having trouble with.
One possible plot could be Goku from the SSJ4 form crossing over into the world of Dragon Ball GT. He could encounter new threats that require his ultimate form to defeat. Maybe he teams up with the other Z - fighters in that universe to face off against a powerful and ancient evil that was sealed away long ago and has now been unleashed.