One interesting concept could be alternate versions of Hogwarts. For example, a Hogwarts where the houses have different characteristics or where the magic taught is more focused on a particular type of magic like dark magic being taught openly (in a more morally grey fanfic). Another concept might be different fates for the main characters. Harry could be a Slytherin instead of a Gryffindor, which would lead to a completely different story arc. There could also be crossovers with other fictional universes in the multiversal fanfiction, like Harry Potter characters interacting with characters from Lord of the Rings or Star Wars.
It expands the story by introducing countless alternate realities. For instance, we might see a world where Voldemort won and how the remaining characters survive and resist under his rule. This creates a whole new set of storylines and character arcs. It also allows for the exploration of different character relationships. Characters who were enemies in the original might be friends in a multiversal fanfic, like Harry and Draco.
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.