They might face the challenge of not being recognized by their past selves or other characters. Also, they could accidentally change things in a way that creates new and more dangerous villains. Another challenge could be dealing with the technology and power levels of the past, which are much lower than what they are used to.
It could be someone like Frieza. Frieza is extremely powerful and evil in the Dragon Ball universe. If he somehow found his way into the Naruto universe or if the fusion character was in the Dragon Ball universe, he would be a great threat. His ability to transform and his cold - blooded nature would make him a tough opponent for the fusion of Goku and Naruto.
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.
The novel," Dimou," could be read in full on many websites, including Xiaoxiang Academy. This novel told the story of how a female protagonist schemed against people's hearts after her rebirth, removing the skin of her enemies and protecting her loved ones. The novel had a total of 557 chapters and was completed.
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.