Siliconera dug up some concept art by character designer Daveed Kaplan depicting Superman as he would have appeared in the game -- and revealing some of the villains that would have been included. Surprise! Lex Luthor is there, in ginormous armor, as are Darkseid, Kalibak, Zod, and Zod's associate Non. You can see all of them as they would have appeared, had they not been banished to the Phantom Zone, at Siliconera.
[Update: The Siliconera post is no longer available, and we have removed our image, at Kaplan's request.]