You can't have corn flakes without the milk and Superman would be nothing without Lex Luthor. Wonder Woman, Batman, The Flash and Nightwing? Who knows, they all have their own nemeses, yet none of them is as iconic as Luthor with exception of the Joker maybe. In any case, Luthor is in Injustice: Gods Among Us and the game will be better because of it.
Check out the latest trailer for Injustice and know that the roster of heroes still totally outweighs the roster of villains at this point. Solomon Grundy and Lex Luthor are strong characters indeed but still relatively weak compared to the lineup of just the Superfriends alone. And we're not even going to mention the badass Cyborg.