5. Mongul
Another physical threat for the Man of Tomorrow is Mongul; a lesser known but extremely formidable alien dictator who, over the years, has used Superman as a gladiator to fight in his arena on Warworld. Conceivably, a Man of Steel sequel could begin with Superman's abduction and imprisonment on Warworld. While I would love to see that, I think this one is less likely due to the similarities between Mongul and Darkseid. Of course changes could always be made...
4. Brainiac
Another fan favorite, the alien android Brainiac has never appeared in a movie. His history has been rewritten so many times, it's hard to nail down exactly what makes him tick. Usually, his plans have something to do with collecting Earth's cities by shrinking and bottling them. I, for one, think that Brainiac is a fascinating character with a terrible motivation. His status in the eyes of fans will probably be enough to get him a shot at Superman, but one has to wonder if this wouldn't be a threat best saved for Justice League 2.