5. Darkseid
Man of Steel showed that it is willing to scale the universe in search of a hero. I think they'd be willing to do the same in search of a villain and they just don't get any more ruthless than Darkseid. Ruler of Apokolips, Darkseid's thirst for infinite superiority and the 'anti-life equation' make him more dangerous than most. With the strength of a God, invulnerability, and omega beams that shoot out of his eyes, physically Darkseid would be able to match the Man of Steel step for step.
How would you introduce him in the movie? All Darkseid wants is unlimited power and the answer to the anti-life equation. What's the anti-life equation? It's a formula that gains the mind control over all other beings. Darkseid spends his time sending his agents all over the galaxy searching for it, including Earth.. Darkseid's agents are defeated by Superman which garner Darkseid's interest. Thinking of him as nothing more than a human in the way, Darkseid's curiosity lingers until he believes part of the answer to his anti life equation lies on Earth. In which he is convinced Superman is protecting. Making his way to Earth via a boomtube, Darkseid sets his eyes on the Man of Steel. The rest is... well, a movie I'd love to watch. As my personal favorite Superman villain, I really hope to see Darkseid in the future. If not in a Man of Steel sequel, maybe in a Justice League movie?