Their fused form might have Goku's spiky hair, but with some of Goku Black's purple - tinted highlights. The body would be muscular like Goku's, but with a more menacing stance similar to Goku Black. The clothing could be a combination of Goku's orange gi and Goku Black's black under - shirt, maybe with some red accents added in for an extra pop of color.
In fanfiction, if Goku Black fused with Goku, it could potentially create an extremely powerful being. This new entity might have a mix of Goku's pure heart and Goku Black's more sinister motives, which could lead to a complex internal struggle. Their combined power could be off the charts, perhaps strong enough to defeat any foe in the Dragon Ball universe. However, it could also cause chaos as their different personalities clash within the fused form.
The plot could involve Goku and Goku Black fusing accidentally. Maybe due to some strange energy anomaly. And then they have to figure out how to control this new form. They might initially be feared by their friends because of Goku Black's reputation, but then they prove that they are on the side of good by using their new powers to protect the Earth from an impending threat that no one else could handle.
It could be Zamasu. Since Goku Black was Zamasu in Goku's body in the original story, perhaps a more powerful and enraged Zamasu could be the main antagonist. He might be jealous of the new fused form's power and seek to destroy it.
In this fanfiction plot, a powerful sorcerer or some sort of interdimensional being could be the cause of the fusion. The fused Naruto - Goku would have an insane amount of power. They could travel at super - fast speeds like Goku and also use shadow clones like Naruto. Their power level would be off the charts. They might end up in different parts of the new world they're in, trying to find each other and figure out what happened, all while being chased by enemies who want to study or destroy their new form.
Maybe it would be a combined form of some of Naruto and Goku's most powerful enemies. For example, if Frieza and Orochimaru somehow combined their powers and knowledge. They would be a very tricky opponent for the fused Naruto - Goku as they would know the strengths and weaknesses of both heroes.
In this fanfiction, Goku and Superman might have an epic battle at first. They could test each other's strength and speed. Maybe Goku would start with his Super Saiyan form and Superman would counter with his heat vision and super strength.
It's really hard to say. Goku has his Super Saiyan forms which give him incredible power and speed. But Superman has his own set of powers like super strength, flight, and heat vision. If they fought, it could go either way depending on the situation and how the writer wants it to end.
The exact situation of Nuwa fusing with the Mythical Zenith Heaven could not be learned from the search results provided. The search results mentioned a novel, I! The Mythical Zenith Heaven, Imprisoning Nüwa at the Beginning, and a novel, Primordial: The Beginning of the War Cries Nüwa, Turns Out I'm the Mythical Zenith Heaven. These novels might involve the plot of Nüwa and the mythical Big Luo, but the specific content and integration method could not be determined. In addition, the search results also included some introductions to the Chinese mythology of Nüwa and related Yangge performances, but there was no mention of the fusion of Nüwa and the mythical Big Luo. Therefore, it was impossible to give an accurate answer as to why Nuwa had fused with the mythological Zenith Heaven.