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.
A Goku and Goku Black fusion in fanfiction could be really exciting. The fusion would inherit Goku's love for fighting and getting stronger, but also Goku Black's ambition. Their combined ki could be a new type of energy that no one has ever seen before. This new fused form might also have new transformations or abilities. For example, it could combine Goku's Super Saiyan forms with Goku Black's Rose form in unexpected ways. It could change the power balance in the Dragon Ball world and open up all sorts of new storylines, like teaming up with other characters to take on new threats or even having internal struggles between the two personalities within the fusion.
It could be insanely powerful. Goku already has incredible power levels, and Goku Black is no slouch either. When they fuse, it's like combining two nuclear reactors. Their power could be so great that they could effortlessly destroy planets just by flexing their newfound power. They might be able to reach Super Saiyan forms that are far beyond what we've seen in the official series, with power levels that are basically off the scale.
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.
I think Zeno would be a really tough opponent. Zeno has the power to erase entire universes, and a Goku and Goku Black fusion, despite being extremely powerful, would have a hard time against such a being with god - like powers.
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.