4. Parasite
There have been a couple different Parasites, but the one usually used is the post-Crisis Rudy Jones. After being dowsed by radioactive chemicals, Jones was able to absorb energy from people, killing them in the process. This became his only means of survival or else he would wither away. When interacting with Superman he absorbs much of Superman's strength, becoming a gigantic monster depending on the artists' interpretation. This usually leaves a weakened Superman having to figure out how to defeat Parasite without giving him more power or dying himself. Parasite would be a good villain because he provides the chance to explore Superman's limitations. In Man of Steel we never got to see a considerably weaker Superman since Snyder was opposed to resorting to Kryptonite so early in the franchise. The possibility of having the Man of Steel up against someone who would not only match him in strength, but make him weaker in the process would be a good move. Considering how Parasite leeches power off Superman, this would also give Snyder a chance to showcase Superman's intelligence in a fight as he'd have to use his wits to defeat him rather than brute strength.