It could be Vegeta. Maybe Vegeta does something really underhanded like stealing Goku's senzu beans or trying to take all the credit for a victory that Goku also contributed to. Goku, being a fair - minded person, would get angry at such behavior.
Well, perhaps in the fanfiction, Goku gets mad when he discovers that someone has been using his name for evil deeds. This would go against his values of justice and protecting the innocent. He might get really angry and go on a search to clear his name and stop the impostor. Or it could be that a new, extremely powerful villain emerges and starts to destroy planets without any remorse. Goku, being the defender of the universe, would be enraged by such senseless destruction and would confront the villain with all his might.
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.
In the fanfiction world where Goku gets Ultra Instinct against Zamasu, the battle is a spectacle. Ultra Instinct Goku is like a different being. His reflexes are on a whole new level. He can sense Zamasu's every move before it even happens. Goku might start by testing Zamasu's defenses with quick jabs. As the fight progresses, he uses his Ultra Instinct to launch a full - scale offensive. His attacks are not only strong but also strategic. He might target Zamasu's weak points that he couldn't reach before. The fight could also involve Goku using Ultra Instinct to break through Zamasu's time - based abilities if the fanfiction has such elements.
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.