There's also a possibility that he meets some long - lost or forgotten characters. Maybe there are some ancient warriors or sages that only existed in the past. Goku could learn a great deal from them. They might teach him about the origin of the Dragon Balls or some other important aspects of the Dragon Ball universe that have been lost over time.
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.
In this fanfiction scenario, Goku could meet Bulma when she was a child. He could befriend her in a different way compared to their first meeting in the original story. Also, he could run into King Kai at an earlier time and start training with him much sooner, which would significantly boost his power levels and change the course of his battles in the future.
Well, he might prevent some of the villains from ever becoming powerful. For example, he could stop Frieza from getting all those power - ups.
In the 'Sora Goes Back in Time' fanfiction, Sora may run into his friends at different points in their lives. He could meet Riku when they were just kids, and see how their friendship started in a whole new light. He might also come across Kairi at a crucial moment in her past, and his presence could change the way she develops as a character. There could also be encounters with other minor characters from the original story, whose roles could be expanded upon in this time - traveling scenario.
She might meet her younger self. This could be a really strange encounter as she tries to understand how she used to be.
Well, he could change a lot of things. Maybe he'd prevent some of the big battles or save some characters who died. For example, he might stop Frieza from destroying Planet Vegeta earlier. It would be a whole new timeline with different outcomes.
He might meet Luke before he turned bad. Percy could try to change Luke's fate, convince him not to follow Kronos. It would be a great chance to save a friend and prevent a lot of the chaos that happened later.
He might meet a younger version of Alfred. Alfred has always been an important figure in the Wayne family and Damian could learn a lot from him, or even try to give him some hints about the future. Also, he could run into a younger Commissioner Gordon. Their interactions could be really interesting as Gordon is such an important ally of Batman.
Well, he could change a lot of things. Maybe he'd prevent some of the big battles or save some characters who died in the original timeline. For example, he might stop Frieza from destroying Planet Vegeta earlier. It would also be interesting to see how his Ultra Instinct powers interact with the past events and characters who haven't seen such power before.
They might face the challenge of not being recognized by their past selves or other characters. Also, they could accidentally change things in a way that creates new and more dangerous villains. Another challenge could be dealing with the technology and power levels of the past, which are much lower than what they are used to.
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.