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 fanfiction, if Goku and Vegeta go back in time, they might change the course of history. For example, they could prevent some of the villains from ever becoming powerful. They may also meet their past selves, which could lead to some really interesting battles or training sessions. And they could introduce new techniques to their past counterparts, making the Saiyans even stronger earlier in the timeline.
Well, they could change the entire timeline. Maybe they'd go back and team up with their younger selves to take on foes that were previously too powerful. They might also influence the relationships between characters. For instance, Vegeta could potentially have a different attitude towards the Z - fighters earlier, which would change the dynamics of their group. It could lead to a whole new set of adventures and battles that we haven't seen in the original story.
Maybe Goku goes back to the time when Planet Vegeta was still intact. He could try to prevent Frieza from destroying it. This would change the entire Saiyan race's fate and might create a whole new timeline with a powerful Saiyan empire.
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.
One possibility could be Luffy changing key events to prevent the losses of his dear friends. This would add a lot of drama and emotional depth to the story.
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.