Krillin would probably be one of the first to be accepting. He's always been Goku's friend and would likely see Goku's inner strength regardless of the physical form. He might even joke about it a bit at first. Vegeta, on the other hand, would be initially shocked and perhaps a bit dismissive. But as he sees Goku's power in the new form, he would gradually come to respect it. Gohan would be very curious and would want to know how this transformation happened and what it means for their future battles.
Goku's reaction could be a mix of shock and joy. On one hand, he has been on Earth for so long without any real family connection to his Saiyan side. When he sees his parents, it would be overwhelming. He would likely be in awe of his father, Bardock, given his father's reputation as a strong Saiyan. And he would be eager to bond with his mother, who he has never known. He might also feel a sense of belonging that he has been missing all his life.
Well, it could be about the Z Fighters from Dragon Ball observing the fan - made scenarios in Death Battle fanfiction. Maybe they are reacting to the battles, power - level comparisons, and unique character match - ups that fans create in these fictional stories.
In this fanfiction, they might have an epic sparring session. Adult Goku could teach Kid Goku some new techniques, like the Instant Transmission. It would be amazing to see how Kid Goku reacts to Goku's Super Saiyan forms.
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.
Obviously, the main characters would be the Z Fighters from Dragon Ball. This includes Goku, Vegeta, Piccolo, and others. Since they are the ones 'watching' the Death Battle fanfiction.
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.