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.
Another plot could involve the villains somehow getting access to the hyperbolic time chamber. They use it to plan and strategize in secrecy, and Naruto and his friends have to figure out a way to stop them without knowing what they've been up to in there.
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 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.
One idea could be Naruto going into the time chamber to train for a new, extremely powerful jutsu that no one has ever seen before. He could emerge with this jutsu and use it to defeat a powerful, new enemy that threatens the Hidden Leaf Village.
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.
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.
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.
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 that Naruto travels back in time and gets adopted by a different clan. For example, the Uchiha clan. This would change his whole upbringing and how he views the world. He might learn different jutsus earlier and have a different relationship with Sasuke.
A cool plot idea could be Minato traveling back to his own childhood. He could then use his future knowledge to train himself more effectively, making himself an even more powerful shinobi at a younger age. This would have a huge impact on the future of the Leaf Village as he would be able to better protect it earlier. He might also be able to prevent some of the early tragedies that happened in the village.