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.
😋I recommend the following novels to you:
1. "The Supreme Traverser: " This novel is a wuxia fantasy novel. The protagonist, Ye Feifan, is reborn in another world. He is extremely talented and can travel through time and space and possess the soul. Coupled with the Heavenly Clone System, it was very interesting and exciting. It was worth reading.
2. "Selling Houses in the Universe at the Beginning": A novel about the infinite universe. The protagonist was a host of a house selling system. As long as he sold his house, he could become stronger. This article had elements such as system style, relaxed style, ridicule style, country war style, and so on. It was very interesting.
3. "Unspecialized: My Skills Can Be Upgraded": This is a novel about supernatural powers. The main character, Gu Nianchen, has traveled through the Psionic World and possesses the basic skills of various classes. The interface is very hot-blooded. You can take a look if you like it.
I hope you like my recommendation. Muah ~
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.
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.