Power and training. Since Goku is known for his power - seeking, his brother in fanfics often has a similar drive. They might train in unique ways or discover new forms of power.
A really interesting 'brother of Goku fanfic' is one where the brother was sent to a different planet. The story details his journey to find his way back to Earth and reunite with Goku, while also facing many powerful enemies along the way.
One possible plot could be that Naruto, as Goku's brother, has to learn the ways of ki while also maintaining his ninja skills. Maybe they team up to fight a common enemy that threatens both the ninja world and the Dragon Ball universe.
If Goku shared spirit with his brother in a fanfic, it could potentially change the dynamics of their relationship and unlock hidden powers. It could also create new challenges and conflicts that they have to overcome together.
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.
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.
Loki would also be a difficult adversary. He's very cunning and could use his tricks and illusions to try and outwit Goku. Goku would need to rely on his senses and ki to see through Loki's deceptions.
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.