Goku might die due to the overuse of his powers. In the series, we've seen him push his limits multiple times. In fanfiction, this could be taken further. He could push himself too far in a fight or while training, and his body just gives out, leading to his death.
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 through a self - imposed death. For example, if he discovers that his very existence is causing some sort of imbalance or danger to the universe, he might choose to end his own life to protect everything he loves. This would be a very noble but also very sad way for Goku to go in fanfiction.
One possibility is that Gine could train Goku in some long - forgotten Saiyan techniques. Since she is his mother, she might know some unique skills that could make Goku even stronger.
A plot could involve Goku being transported to a future where all his friends are dead and the world is in ruins. The unknown circumstances that led to this and the feeling of helplessness could make him scared. He then has to figure out how to go back in time and prevent this future from happening, all while being afraid of what he might find out about how this came to be.
In some fanfictions, Goku and Raditz could form an unlikely alliance if they were faced with an even greater threat. For example, if a new and extremely powerful enemy emerged that threatened not just Earth but the entire Saiyan race. Raditz, who initially came to Earth with the intention of conquering it, might see the need to team up with Goku to save their home planet. Goku, being the good - hearted Saiyan he is, would be open to the idea if it meant protecting his loved ones.
One possible plot could be that Android 21, with her unique powers, teams up with Goku to face a new and powerful enemy. They might have to combine their strength and fighting techniques to defeat this foe.
In this kind of fanfiction, Goku might lose his family. For example, his friends and loved ones could be killed by a powerful new enemy. This would leave him in a state of deep despair and loneliness, and he would have to find a way to carry on without them.
In this fanfiction scenario, Goku meeting Gine could be really interesting. Gine is Goku's mother, and in a fan - made story, their meeting might involve Gine finally seeing how strong her son has become. Maybe she'd be proud of all his achievements in protecting the Earth and the universe. It could also be a very emotional encounter as Goku has grown up without her presence for most of his life.
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.
One possible change is his power level. Fanfictions often give Goku new power - ups that are even more extreme than in the official story. For example, he could gain a 'Ultra - Ultra Saiyan' form that is many times stronger than the regular Super Saiyan forms. His appearance in this form could be very different, with longer hair, more defined muscles, and a new color to his aura. Another change could be in his family life. Maybe he spends more time with his family, teaching his sons and daughter new fighting techniques that he created himself. This would show a more domestic side of Goku that we don't see as much in the original series.
It would definitely change the dynamics of the story. Maybe their relationship would bring new challenges and opportunities for the characters.
Another potential rival could be a young, prodigious martial artist from a new school of martial arts on Earth. This character could be jealous of Goku Jr's fame and constantly try to outdo him in battles and in the eyes of the public. He might use unorthodox techniques that Goku Jr has to quickly figure out how to counter.