It could be a new, self - created villain. This villain might have been training in secret for years, specifically to defeat Goku. He could have studied all of Goku's moves and weaknesses and then launch a surprise attack that causes Goku to get hurt.
Often, in 'Goku Harem Fanfic', Goku becomes more aware of his romantic side. He might be a bit more in tune with the emotions of the female characters around him compared to his original portrayal in the Dragon Ball series, where he was mostly focused on fighting and training.
His anger often manifests physically. His muscles might bulge more than usual, and his hair could stand on end even more in his Super Saiyan forms. His ki, or energy, also becomes more visible and intense. It's like a red - hot aura surrounding him.
His social skills might be different. Instead of the somewhat naive Goku we know, he could be more aware of social norms. Bulma could teach him about proper behavior, but at the same time, his Saiyan impulsiveness would still shine through at times. So he might be a strange mix of a well - mannered person in some situations and a wild fighter when provoked.
His character would develop a more strategic side. Beerus is a very strategic fighter, and Goku, being raised by him, would learn to think more before acting in battles. He would also become more aware of the power hierarchy in the universe at a younger age, which could make him more cautious or more ambitious, depending on how you look at it. Additionally, his Saiyan pride might be tempered by Beerus' teachings, making him more of a team player in some ways.
Goku's powers are on a much higher scale. His ability to destroy planets as a Super Saiyan makes him far more powerful than Naruto at his strongest. Naruto's jutsu are mainly focused on the ninja world, while Goku's powers are universal.
In certain fanfictions, Goku's depression is manifested in his dreams. He could have nightmares about his failures or the destruction he couldn't prevent. These dreams haunt him during his waking hours, making him more withdrawn and less likely to engage in his normal activities.
In some fanfictions, Gohan and Goku might have a series of conversations that open up new levels of understanding between them. Gohan could share his thoughts and feelings about his life and future, and Goku would listen and offer his own insights. This kind of communication can really strengthen their bond.
Goku's fighting style might become more fluid. Female characters are often depicted as having more grace in movement, so Goku as a girl could incorporate some elegant dodges and strikes into her already powerful combat techniques.
In such fanfictions, Goku might develop by becoming more of a mentor to Gohan. He could teach Gohan new fighting moves and also how to control his emotions better during battles.