In "Man of Steel", at the beginning we see Jor-El on Krypton steeling this weird magical skull called the codex. Just before Kal-El's (Superman's) space ship blasts off, Jor-El downloads the Codex into Kal-El, which begins the start of the movie.
We find out later that the Zod wants the Codex because he can re-create Kryptonian life on earth because the Codex holds the make-up of every Kryptonian that will ever be born.
I can see this working into a potential Man of Steel sequel; Brainiac comes, and informs the world (even Lex Luthor) that the Codex lives in Superman, and causes Lex to create a terrifying threat for Superman (Bizarro? Mongul? Doomsday?) .
I really don't think that Zack Snyder put this weird Codex in Man of Steel inside of Superman unless he planned a consequence in the future (like the destruction of Metropolis in Man of Steel); and at the rate Snyder is going with this "Action and Result" formula, the man will eventually sweat Gold.
What do you think? will the Codex return?