Well, Vegeta would be a likely character. He might be initially suspicious of the female Goku. Maybe he thinks she's a threat or just can't believe there's another Goku - like being. He could be there to add some comical moments as he tries to understand the situation.
In general, it's a bit of a toss-up. The strength of Manga Goku and Anime Goku can vary depending on the specific arcs and battles. Sometimes, it might seem like one is stronger in a particular situation, but that could change in another.
Another possible main character could be the villain that forces the two Gokus to meet. This villain could be a new, super - powerful threat that the two Gokus need to deal with. Their meeting becomes a crucial part of the story to defeat this powerful foe.
An example of a continuation of the necklace was as follows:
When the protagonist of the maupassant bought the necklace in the mall, he did not know that there was a terrible story behind the necklace.
A few days later, his wife found the necklace and felt uneasy. She confessed the secret behind the necklace. He was shocked and sad because he had always thought that the necklace was just an ordinary gift, but he had never thought that it would cause him so much pain.
His wife passed away very quickly, leaving behind a heart-wrenching maupassant. He felt that he had lost everything, the most important thing in his life.
However, he did not give up. He decided to find out the truth about the necklace and uncover the secret behind it. He spent a lot of time and energy investigating every detail and finally found out that the necklace was the murderer.
It turned out that the maker of the necklace was a wealthy businessman who used his wealth and status to deceive his wife and put her in danger. He planned to use the necklace to murder his wife, but because his plan failed, the necklace was accidentally discovered by Maupassant.
He realized that he had lost everything, but he had also become stronger and braver through this experience. He decided to pursue the legal responsibility of the murderer and return the necklace to his wife's family. He missed his wife dearly but also realized that the most important things in life were freedom and justice.
One of the characters who could betray Goku is Piccolo. Although they've been allies for a long time, in fanfiction, things can change. Perhaps an ancient evil force takes control of Piccolo's mind and makes him turn on Goku. Or it could be Krillin. He might be influenced by an outside force that convinces him Goku is becoming too dangerous. Also, don't forget about the Kais. If there's some sort of power imbalance or a threat they perceive from Goku, they might betray him to maintain the order of the universe in the fanfiction.
Well, it could be about Goku Jr learning some of Goku's most powerful moves. Maybe Goku teaches him the Kamehameha. They might also go on an adventure together, like fighting a new and powerful enemy that neither could face alone.