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.
One story could be about a group of activists who managed to rescue a group of animals from a testing facility. They worked secretly, coordinated their efforts, and finally freed the poor animals, giving them a chance at a normal life in a sanctuary.
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.
Birds are also good for stories. They can symbolize freedom. For example, eagles are often used to represent strength and freedom in stories. Their ability to fly high in the sky gives a sense of aspiration. Sparrows, on the other hand, can be used to show the common and the ordinary, yet they can also have important roles in stories about community and togetherness.
The animals are often depicted as calm and gentle. For example, the sheep are usually shown as meek and mild, while the donkey is portrayed as a patient and helpful animal that carried Mary to Bethlehem.
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.