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.
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.
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.
In a Gt Goku time travel fanfiction, Gt Goku might travel to a time when the Namekians were first attacked. He could help them defend their planet more effectively. This would not only save the Namekians but also have a big impact on the Dragon Ball story. Because the Namekian Dragon Balls are crucial. If they were never stolen in the first place, many events in the story would be different. And Gt Goku's actions could also lead to a different relationship between the Namekians and the Earthlings.
Well, the plot could be that Goku and Android 18 are sent back in time by a powerful enemy's time - warping device. They end up in a time when the Saiyan race was still thriving on Planet Vegeta. Goku, being a Saiyan himself, is conflicted about interfering with the past. Android 18, on the other hand, is more interested in finding a way back. But as they stay there, they start to realize that there are things they can do to make the future better, like preventing certain evil Saiyans from doing terrible things that would later have far - reaching consequences for the universe.
Goku is, without a doubt, the main character in these fanfictions. But other characters like Gohan and Goten can also be part of the story. Gohan might accompany Goku on his time - traveling adventures, or Goten could be a key factor in a future - related plot. Additionally, the Kais, especially the Supreme Kai, could be involved. They have knowledge about time and the universe, so they could play important roles in guiding Goku during his time travels.
One interesting concept could be Goku traveling back in time to train with his past self. This would allow him to learn new techniques and gain more power. Another idea might be Goku traveling to the future to prevent a terrible disaster from happening. He could use his skills to change the course of events.
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.