One idea could be a fanfic where Goku travels to a parallel universe and has to fight an evil version of himself. Another could be a story where Goku has to train a group of young Saiyans on a new planet. Or perhaps a fanfic where Goku discovers a new form of energy and has to learn how to control it.
One idea could be a crossover where Goku accidentally travels to the Naruto universe and meets Sakura. They could team up to fight a common enemy that is causing chaos in both their worlds. Another could be a story where Sakura somehow gets the power to travel to different universes and ends up in Goku's world. She then trains with Goku to become stronger.
One idea could be a Goku crossover with Superman. They could have an epic battle to test their strength at first, and then realize they have a common enemy and team up. Another idea is a crossover with Naruto. Goku could teach Naruto some of his ki techniques, and in return Naruto could show Goku some of his ninja skills. A third idea is a crossover with Harry Potter. Goku could visit Hogwarts and accidentally cause chaos with his superhuman abilities, but then end up helping the students fight off a dark wizard.
One idea could be that Goku arrives in the Pokemon world by some sort of interdimensional rift. He might be confused at first but then start to explore the concept of Pokemon battles. He could use his martial arts skills to train Pokemon in a unique way, like teaching them special combat moves. For example, he could train a Pikachu to use a move similar to his Kamehameha, but in a more Pokemon - friendly version, like an electric blast with a similar charging motion.
Perhaps a fanfic where Chi - Chi discovers a lemon - themed cooking competition and she enters with Goku as her helper. They face various challenges related to using lemons in their dishes and in the process, their relationship deepens.
Maybe a fanfic where Goku discovers Turles has a different side to him. Turles could be in a situation where he has to protect something or someone innocent, which makes Goku see him in a new light. They then have to work together to keep this innocent thing safe from other villains in the Dragon Ball universe.
A possible plot is that Android 21 has a unique ability that Goku needs to defeat a powerful, upcoming enemy. So he seeks her out and tries to convince her to help. Along the way, they face challenges like other androids who are still loyal to the old ways. Another idea is that Android 21 creates a clone of Goku by mistake while trying to study his power. This clone has some of Goku's power but also some of Android 21's android - like features. They then have to deal with the chaos that the clone causes.
One idea could be a crossover tournament where Naruto and Goku both participate. They start as rivals but gradually come to respect each other's skills. Another could be that they have to team up to face a common enemy from both of their universes combined. Maybe a powerful demon or an evil entity that has the ability to use both ninja techniques and ki powers.
One idea could be a crossover where Goku and Naruto are brought to a strange new dimension together. They have to team up to fight a powerful villain that neither of their worlds has ever seen. Their different fighting styles, Goku's ki - based attacks and Naruto's ninjutsu, would make for some really cool battle scenes.
One idea could be that Goku has been away on a long and difficult training journey. The reader, who is an old friend, hears of his return and goes to meet him. They then share stories of their adventures since they last saw each other, and Goku shows off some of his new techniques.
One idea could be that Goku accidentally travels to the RWBY universe during a sparring session that goes wrong with Whis. He could be mistaken for a powerful Grimm at first, causing chaos. Then, he teams up with Team RWBY to fight a new super - strong Grimm that threatens all of Remnant.