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.
Goku Black is a major character. He brings his Saiyan strength and his twisted sense of justice from his own timeline. Android 21, on the other hand, is a character full of mystery. In fanfiction, she often has her own motives and goals that interact with Goku Black's, whether it's to gain more power or to find a place in the world. Other characters might also be involved, like some of the Z - Fighters who could be either allies or enemies in the story depending on the plot.
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.
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.
Chi - Chi might be one of the first to support them. Despite her past relationship with Goku, she could see how happy Goku is with Android 18 and decides to be happy for them. Also, Krillin would likely support the marriage. He has a deep friendship with both Goku and Android 18. He knows Android 18 well and trusts Goku, so he would be all for their union.
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.
Android 21 and Goku are the main characters. Android 21 with her android intelligence and power is a key figure. Goku, with his super Saiyan forms and his indomitable spirit, is equally important. Their personalities are so different - Goku is more straightforward and pure - hearted, while Android 21 has a more complex and sometimes morally ambiguous nature. These differences create great potential for conflict and growth within the fanfic.
One idea could be that Goku and Android 21 team up to face a new and extremely powerful villain. This villain could be from another dimension and has the power to corrupt the minds of other fighters. Goku, with his pure heart and Android 21's intelligence, find a way to defeat the villain.
Another possible storyline is that Android 21, who is usually seen as a bit of an outcast, finds acceptance in Goku's group. Goku encourages the others to give her a chance. They then go on adventures together, with Android 21 constantly evolving not just in power but also in her understanding of the world around her.