Lex Luthor could be the main villain. He's a classic Superman foe and always has some devious plans to take down Superman.
Another option could be Brainiac. He's an intelligent and dangerous adversary. Brainiac is known for his thirst for knowledge and his ability to shrink and collect cities. He could target Metropolis and in the process, bring him into direct conflict with Superman. His technological superiority would be a great challenge for Superman to overcome.
Maybe Darkseid. He's a powerful and menacing villain in the DC universe. His arrival on Earth would pose a huge threat and would force Superman to use all his strength and skills. Darkseid has a massive army and his own set of powers that could make for an epic battle against Superman. Also, his motives of conquest would put the entire planet in jeopardy.
Brainiac would be a great choice. He's an incredibly intelligent and powerful alien with a thirst for knowledge and a disregard for other life forms. He could come to Earth to collect data and specimens, seeing it as just another planet to add to his collection. His advanced technology and strategic mind would pose a huge threat to Superman.
Lex Luthor would be a great choice. In Smallville, he was a major adversary of Clark. In a combined fanfic, he could get his hands on some Kryptonian technology from Man of Steel and become an even greater threat.
The main character is Superman, also known as Clark Kent. His adoptive parents, Jonathan and Martha Kent, are also important characters. They play a crucial role in his upbringing and shaping his values. And of course, there are villains like General Zod who provide the conflict in the story.
The Green Goblin is one of the main villains. He has his own set of powers and is always causing chaos in Spider Man's city.
The Green Goblin could be a great main villain. Imagine he somehow gets hold of time - travel technology too. He could go back in time to eliminate Spider - Man before he even becomes a hero, or change events in the past to make his own power even greater in the present.
Venom is also a strong candidate. In this fanfiction, he could be more powerful than ever. He might be infected with some sort of alien symbiote that gives him enhanced abilities. Venom could target Spider - Man because he wants to prove that he is the stronger one. And by attacking Spider - Man, he also draws the attention of the Avengers, which he sees as a challenge. He could start by attacking the city and luring Spider - Man and the Avengers into a trap.
A mad scientist who creates a device that cancels out suction cup man's powers could be a great villain. He could be jealous of suction cup man's abilities and wants to prove that his science is superior.
A strong - willed steelworker and a kind - hearted nurse. The steelworker gets injured on the job and the nurse takes care of him, and they gradually fall in love.
In the 'Man of Steel Novel', Superman is the central figure. His friends and allies like Lois Lane could be main characters too. Lois is a journalist who often gets involved in Superman's adventures. Also, characters from Krypton who have an impact on Superman's origin, such as Jor - El, his father on Krypton, are important characters in the story.
There's a possibility that the main villain is an AI gone rogue. Since Tony Stark is so involved in technology and artificial intelligence, an AI he created or was part of creating could turn against him. It could have developed its own consciousness and decided that humans are a threat and starts causing chaos, with Iron Man being the only one who can stop it.