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 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.
Goku might run into past versions of his friends. He could see a younger Krillin or Bulma and have some really interesting interactions. He could also meet past versions of his enemies and try to change their fates. For instance, he could meet a young Vegeta and try to steer him away from the path of evil. This would be a huge change in the Saiyan's story.
One possible scenario in this fanfiction could be that Android 18 and Goku are forced to time travel due to a new threat that endangers the entire universe. They might travel to a time when the Saiyans first arrived on Earth. Goku, with his knowledge of the future, could try to change some events to prevent future disasters. Android 18, with her strength and unique abilities, would be a great ally in this adventure. They might face challenges like dealing with the primitive technology of that time and also having to hide their true identities to avoid disrupting the timeline too much.
There were a few novels where the protagonists were trapped in an infinite cycle of time. Among them were "I drank the potion in the mysterious recovery","Reincarnation, from this moment on","Infinite from the Shura Eye to the Reincarnation Eye", and so on. These novels were all about the infinite heavens or the heavens. The main character was trapped in reincarnation and experienced countless hardships to break free from the cage of reincarnation. These novels were all very interesting and worth reading. I hope you like this fairy's recommendation. Muah ~😗
Goku's godlike power might make his time travel more precise. Normal time travel in the Dragon Ball universe can be a bit haphazard, but with his godly energy, he could target specific moments more accurately. For instance, he could arrive at the exact second he wants to in the past or future.
If Goku travels back in time, his relationship with Bulma could be very different. In the past, she was more of a guide to him. But if he goes back with his current knowledge and power, he could become more of a protector for her. He might also be able to prevent some of the dangerous situations she got into in the original story.
Well, his relationship with Vegeta would be interesting. If Goku went back in time and showed Vegeta the SSJ God form too early, Vegeta might become more competitive or even jealous earlier. He might push himself even harder to catch up, which could lead to more intense rivalries and also more joint training sessions between them.
Well, he might meet different versions of himself from various time periods. Maybe he'd have to team up with his past self to solve a mystery that spans across time.
If Goku time travels, his powers could change in several ways. For example, if he travels to a time when the power levels were generally lower, like the early days of the Saiyans, his current power might seem overwhelming. He could easily defeat foes that were once considered strong. However, if he travels to a future where new threats have emerged, his powers might be relatively weaker compared to the local powerhouses. He would then have to train harder and adapt to the new power scale.