6. Brainiac
Brainiac is not just a name of a character from comics and TV, the name "brainiac" itself has crossed over into the popular mainstream lexicon as another descriptor for "highly intellectual". But, Brainiac himself, always a perennial fan-favorite, and a 12th-level green-skinned world-conquerer from the planet Colu is practically begging for a big-screen debut. Having toiled in aborted film scripts and played via human proxy by Smallville's limited budget and understanding of comics and casting, Brainiac would not just pose a threat to Metropolis, but to the planet itself, as Brainiac often resorts to stealing the major city of a world he is conquering, shrinking it down to miniature size and placing the city and its inhabitants into a glass bottle to add to his collection on his tentacled ship. Superman will often find he has a far deeper connection to Brainiac, for in mostly all incarnations, Brainiac is responsible for an attack on Superman's former home world of Krypton, where Brainiac stole and shrunk Kandor, the metropolis of its day in Krypton. Not only would Kal-El's battles with Brainiac be highly charged action set pieces with the backdrop of outer space and world-in-peril be amazing, but would also bring a humanizing virtue to Superman's quest to rescue and resize his former fellow citizens, finally having a piece of his home world so close, but not being able to save it as it sits before him, the action and the drama practically writes itself for a MoS sequel. Then there's always the possibility of Kal shrinking down to visit Krypton in the form of miniaturized Kandor for a threequel (or, dare I say, a "World of New Krypton"), but let us not get too far ahead of ourselves.