A fanfiction might explore the idea of Bardock seeing Goku's life through a time - space rift. He witnesses all of Goku's battles and the friends he makes. Bardock is filled with pride but also realizes that Goku has a different path than the traditional Saiyan way. This makes him question the Saiyan culture he once knew. Meanwhile, Goku starts to have strange dreams about his father, which leads him to search for more about his Saiyan heritage.
In some fanfictions, Goku's death might be a sacrifice to save the universe. He could be facing an enemy so powerful that the only way to stop it is by giving up his own life. This would be a very emotional moment for his friends and family, like Chi - Chi and Gohan.
One common theme is empowerment. Since Goku is already a very powerful character, in these fanfics where she's female, it often emphasizes her strength even more, showing that gender doesn't limit her abilities.
At first, Caulifla might be a bit intimidated by Goku's power. But as they train together, she would start to understand his techniques and philosophy of fighting. Goku would also learn from Caulifla's unique approach to battles. Their relationship could evolve into a deep bond where they rely on each other in tough battles. For example, Goku could save Caulifla from a near - death situation, and in return, Caulifla could come up with a creative battle strategy that helps Goku against a difficult opponent.
He could die in battle. Maybe a new, extremely powerful villain emerges who has the ability to nullify Goku's powers or is just so strong that Goku's attacks have no effect. After a long and intense fight, Goku is defeated and dies.
Another way is that Goku could use the Spirit Bomb. He could gather energy from all living things on Earth and nearby planets. Android 21, being an artificial being, might not be able to withstand the pure energy of the Spirit Bomb. However, she could try to absorb the energy, but if Goku can overload her absorption capabilities, it would be his victory.
In a fanfiction scenario, Goku and Android 21's fight could attract the attention of other Z - fighters. Vegeta, for example, might be reluctant to help Goku at first but then realizes the danger Android 21 poses to the entire planet. Goku could also try to reach out to Android 21's good side, as he has a history of trying to redeem his enemies. This could add an interesting moral dilemma to the story, whether to fight or try to save her.
A possible plotline could be that Bardock, in the afterlife, is able to communicate with Goku in his dreams. He warns Goku about an upcoming threat that is even more powerful than the villains he has faced so far. Goku then starts a new training regime in preparation for this threat, and along the way, he discovers more about his father's past and the true potential of the Saiyan race.