2. Kara Zor-El
Perhaps the most subtle reference in Man of Steel hints at the possible existence of Superman's cousin, Kara Zor-El (also known as "Supergirl"), in the DC Cinematic Universe. When Clark Kent first enters upon the Kryptonian scouting ship that would become his Fortress of Solitude, he finds a series of "cryo-tubes". One of the tubes seems to be occupied by a mummified corpse, but the camera seems to purposely linger on another tube that has been left open and is apparently empty. Although there is no discussion about this empty tube, we can certainly assume that one of the ship's Kryptonian inhabitants exited the ship whilst still alive, which means there's another Kryptonian out there somewhere. Perhaps she could come to Earth and live with Clark's mother and learn to live there by virtue of Clark's mentoring and become a heroine unto herself in a future movie - perhaps to assist Clark in the final battle against Darkseid, like she has done in comics and animated features.