Another possibility is a mad scientist who creates powerful androids. These androids are sent to fight Goku and Power Girl. The androids could have unique abilities that pose a great challenge to our heroes, forcing them to come up with new strategies and power - ups to defeat them.
Often, it could be some sort of super - powered being from a different dimension. Maybe a powerful wizard who wants to take over both their universes.
Some fanfictions might feature a group of evil Saiyans as the antagonists. They are jealous of Goku's power and see Power Girl as an obstacle in their path to domination. They would try to eliminate them both.
Darkseid can also be a common antagonist. His goal of universal domination means that he would see Superman and Power Girl as threats to his plans, and he would send his minions or come himself to fight them.
Darkseid could be a main antagonist. His thirst for power and control could put him in direct conflict with Wonder Woman and Power Girl. He might see their combined strength as a threat to his plans of universal domination.
Often, it could be new villains created specifically for the fanfiction. Maybe a powerful android created by the same scientist who made Android 21 but with a more evil intent.
Well, clearly Goku from Dragon Ball and Power Girl from DC Comics are the main characters. Goku is known for his Saiyan heritage and his ever - increasing power levels through training. Power Girl, on the other hand, has superhuman strength, flight, and various other powers.
One type of antagonist is a mad scientist - type character who wants to experiment on Goku and Fasha to understand their Saiyan powers. This character might create all sorts of dangerous monsters to try and capture them. Another could be a powerful demon from another dimension who is drawn to their power and wants to consume it to become even more powerful.
The Saiyans could also be antagonists. If they discover Black Goku's true nature and his alliance with Zamasu, they would likely oppose them to protect their race and the universe.
It could be some of the more powerful villains from both universes combined. Maybe Frieza and Orochimaru team up. Frieza's lust for power and Orochimaru's desire for immortality and knowledge would make them a formidable pair. They would target Naruto because of his potential and Goku because of his strength, aiming to either destroy them or use them for their own evil purposes.
One possible main antagonist could be Frieza. Imagine if Frieza somehow found his way into the Naruto universe. He would be attracted to the chaos and power struggles there. Goku and Naruto would have to team up to stop him from taking over the Naruto world. Frieza's cold - blooded nature and his desire for power would put him at odds with our heroes. His powerful abilities would be a huge threat to both Naruto and Goku, especially since Naruto is still learning about the new powers Goku is teaching him.
In some fanfictions, they might explore different universes together. Goku is used to traveling between different realms, and Power Girl could be intrigued by the new places. They could have adventures and learn from each other's cultures and powers.
Cell could also be an antagonist. He has an interest in powerful Saiyan genetics. The baby, being related to Goku, would be of great interest to him. He might try to absorb the baby to gain even more power, creating a big conflict for the heroes to deal with.