His magic might become more physical. Instead of just waving a wand, he could use ki to direct spells. For example, his Patronus charm could be charged with ki energy, making it stronger and more tangible. Maybe his spells would be instant like Goku Black's attacks, no need for long incantations.
Goku's ki - based powers could be seen as a parallel to the magic in Fairy Tail. His attacks like the Dragon Fist could be considered similar to some of the dragon - slayer magic in the series. When he uses his ki blasts, the mages in Fairy Tail might initially mistake it for a very powerful and unknown type of magic. Goku could also learn to sense ki in a way that helps him detect magic users and their power levels more easily.
Honestly, comparing the strength of anime Goku and manga Goku is tricky. Both versions have their unique strengths and challenges depicted differently, making it hard to make a clear-cut comparison.
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.