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.
With his friends like Krillin and Yamcha, Goku's time travel could strengthen their bonds. He could use his future knowledge to help them train better and avoid some of the near - death experiences they had. For example, he could stop Krillin from getting killed by the villains earlier, which would make their friendship even deeper.
In terms of his relationship with his enemies, if Goku travels to a time when they were not yet as powerful or evil, he might try to change them. For instance, he could go back in time to meet Vegeta when he was still a full - fledged villain. Goku could try to show Vegeta a different path, which would be a very different dynamic from their initial encounters 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.
His relationship with Vegeta would also change. If Goku goes back to a time when they first met, he could try to befriend Vegeta right away instead of being enemies first. This would change the entire dynamic of their rivalry. They might become allies much sooner and train together, which would lead to both of them becoming even stronger at an earlier stage.
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.
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.
Their relationship could become more of a teacher - student dynamic. Goku could teach Android 18 about the ways of the Saiyans and their fighting techniques from different eras during the time travel. Android 18, in turn, could show Goku some of her android - specific skills and how to use them more effectively. This exchange of knowledge could bring them closer and make them a more formidable team.
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.
At first, they might still be competitive. But as they face the challenges of the different time periods together, they could start to respect each other more deeply. For example, if they are in a time when they are much weaker, they would have to rely on each other's skills more.
One possible fanfiction could be Gt Goku traveling back in time to prevent the destruction of Earth in an earlier timeline. He could use his knowledge from the GT era to stop a powerful enemy that was previously victorious.
His relationships with his guild members would also change. He could prevent some of their deaths or mistakes. For example, he might be able to stop Klein from getting separated from the group at the start of SAO. This would lead to a different dynamic within the guild, with more camaraderie and a sense of unity.
If Goku travels back in time as a Super Saiyan in fanfiction, his relationship with his past self would be complex. His past self might look up to him as a hero and a mentor, but at the same time, there could be some envy or confusion. With his Saiyan comrades, he could become a leader figure much earlier, changing the power dynamics among them. Also, his interactions with Earthlings would be different. They would be more fearful or more in need of his protection, depending on how he uses his power in the past.