In a 'Goku in Another World' fanfiction, he could enter a world where there are no other fighters. He then starts to train the local people to become strong like him. This could lead to some interesting storylines as he tries to instill his values of determination and hard work in them, and also has to deal with the local politics and power struggles.
Perhaps Goku lands in a world where the inhabitants are all pacifists. He has to find a way to protect this world from an external threat without using his usual brute force, which could lead to him developing new skills. Also, he might find himself in a world with a different concept of time, which messes with his ability to power up in his usual way.
One idea could be that Goku and Goku Black are forced to team up against an even greater threat from another universe. They start to understand each other's fighting styles during the battle and gradually develop a grudging respect for one another.
In some versions, the female Goku could be part of a prophecy. The two Gokus meet, and at first, there is some confusion and misunderstanding. But as they get to know each other, they discover that they are key to fulfilling the prophecy. They combine their powers and defeat an ancient evil that was threatening both of their worlds.
One idea could be a story where Bulma creates a new device that accidentally sends Goku and her to a parallel universe. There, they have to work together to find a way back home while facing new and strange enemies. Another could be a time - travel plot where they go back in time to before Goku met Chi - Chi and Bulma realizes she has feelings for him. The third idea is a story where Goku and Bulma get trapped on a deserted planet and must rely on each other's skills to survive and escape.
One idea could be a crossover universe where Goku somehow ends up in the Naruto world and meets Sakura. They could team up to fight a new common enemy that is causing chaos in both their worlds. Maybe this enemy has the ability to steal energy like the Saiyans but also uses genjutsu like the ninjas.
One plot idea could be that Baby takes over Goku's body and tries to use his power to conquer the universe in a more subtle way. He might pretend to be Goku at first and manipulate his friends into doing his bidding without realizing it.
Maybe there could be a story where Android 18 gets sent to a different dimension and Goku and Vegeta have to follow her to bring her back. In this dimension, the rules of power and fighting are different. They all have to adapt quickly. Goku and Vegeta might learn some new skills from Android 18 as she has a different way of approaching combat situations due to her android nature.
One idea could be that Zoro ends up in a world full of magic. He has to learn to adapt his sword skills to combat magical creatures. Maybe he teams up with a local wizard who helps him understand the new world's rules while he teaches the wizard about honor in battle.
One idea could be that the Akatsuki members are transported to a world full of magic. They have to adapt their ninja skills to this new environment. For example, Sasori might find new materials to create his puppets, and Deidara could be intrigued by the different ways to create explosions in this magical world.
One idea could be Naruto arriving in a world full of magic, and he has to learn to combine his chakra skills with magic. For example, he might use his Rasengan with a magical spell to create a super - powerful attack.
Another plot could be that Vegeta lands in a world where time moves differently. He might age at a much faster or slower rate. This would add an extra layer of complexity as he tries to figure out how to get back to his own world while dealing with the changes in his physical state and the relationships he forms in this new world.