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.
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.
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.
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.
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.
Often, Goku with Ultra Instinct is more focused. He's not as carefree as usual. He's fully concentrated on defeating Zamasu. His determination is palpable.