Kamen Rider SIN and SHF were both characters in the Kamen Rider series. However, they had different backgrounds and storylines.
SIC was one of the characters in the Masked Rider series. It was a fictional creature created by Ishimori Shotaro who coexisted with humans and possessed superhuman strength and abilities. The main purpose of the SIC is to protect mankind from the threat of evil forces. It played an important role in the story and fought with other masked knights.
In contrast, SHF was another character in the Kamen Rider series. It was a fictional creature created by Shoutarou Ishimori that coexisted with humans but was more powerful than them. SHF's goal was to replace humans as the dominant existence of the world. It played an important role in the story and fought with other masked knights.
There are some key differences between the SICs and SHF in terms of storylines. The SICs 'story revolved around protecting humans while the SHF's story revolved around replacing humans. In addition, the battle between the SICs and the SHF was also an important difference between them. The SIN and the other Masked Riders fought together to protect humans while the SHF fought alone to replace humans as the dominant existence of the world.
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.
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.