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.
Perhaps an ancient evil that has been dormant could be awakened by Black Goku and Zamasu's actions. This evil force then becomes the main antagonist, and Black Goku and Zamasu have to either team up with their former enemies or find a new way to defeat it.
Another option for the main antagonist could be Beerus. As a God of Destruction, he wouldn't be happy with Zamasu and Goku Black causing chaos. Beerus is extremely powerful and has the authority to deal with threats to the universe. He might see their actions as a direct challenge to his power and order. If he gets involved, he could completely change the dynamic of the story, as his power level is on a whole different scale compared to most characters in the Dragon Ball universe.
A common theme is power - seeking. They are often depicted as constantly striving for more power and ruling over the universe. Another theme is their unique bond, whether it's a friendship or a more sinister partnership.
Another option could be Mr. Sinister. He might see potential in Rogue's powers and try to manipulate her. He could also see Nightcrawler as an obstacle in his plan to use Rogue. So he creates various schemes to drive a wedge between them and make Rogue come under his control.
The Goku Black x Zamasu comic tells a story of chaos and power plays. Goku Black and Zamasu have their own agendas, and the comic follows their clashes and how it affects the universe. There are lots of action-packed scenes and strategic showdowns.
One popular 'goku black x zamasu fanfiction' is 'The Dark Alliance'. It delves deep into their partnership and the chaos they create in the universe. Another interesting one could be 'The Merged Power', which explores what would happen if they combined their powers in different ways.
One idea could be exploring their relationship in an alternate universe where they succeed in their plans. Another could be a story where Goku Black has a change of heart due to his connection with Zamasu and tries to stop their destructive ways. Maybe a plot where they face a new enemy that forces them to reevaluate their alliance.
One idea could be that they discover a new realm together and have to face an unknown threat there. Their combined power might be put to the test in ways they never expected.
It's an intense battle between two powerful characters with complex motivations and lots of action.