In the manga, Goku managed to kill Zamasu by combining his strength with Vegeta and performing a joint attack. They also exploited Zamasu's weaknesses to gain the upper hand.
A common theme is power and corruption. Since both characters are power - hungry in different ways, many fanfictions explore how their thirst for power corrupts them further. For example, they might go on a rampage across different universes to gain more power.
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 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 possible plot is a time - travel scenario. Goku Black and Zamasu go back in time to try and change the course of history to make it easier for them to achieve their goals. They might target events like the Saiyan - Earthling alliance or try to prevent Goku from ever becoming a Super Saiyan. This would create a lot of chaos and interesting interactions with past versions of characters.
One popular 'goku black and zamasu fanfiction' is 'The Dark Ascension'. It delves deep into an alternate universe where Goku Black and Zamasu's plans reach even more terrifying heights. Another great one is 'Fusion of Darkness' which explores a scenario where they manage to fuse in a different way than in the original story.
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.
You could combine the powers of Zamasu and Goku Black into a new hybrid - like character. This character could have Zamasu's godly essence and Goku Black's Saiyan fighting abilities. For example, they might be able to use ki blasts with a divine touch that not only damages physically but also weakens the spirit of the opponent.
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.
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.