3. Cyborg
Like Booster Gold, Cyborg was alluded to in the Man of Steel movie - though not quite as subtly. S.T.A.R. Labs make a couple of very blatant appearances in the movie, most blatantly when Dr. Hamilton (also a character from the comics) is very briefly identified as an employee of the company. S.T.A.R. Labs are the people responsible, in the comic books, for the transformation of Victor Stone in to Cyborg after an accident destroys much of his body. He becomes a powerful superhero with an internal armament that makes him a formidable ally and he has become a prominent member of the Justice League. He is another character who has also seen live action in a number of appearances on Smallville and having him as part of a movie universe Justice League would be pretty cool. Given that S.T.A.R. Labs were clearly meant to be noticed in the Man of Steel movie, it would seem DC and Warner Brothers purposely included them to hint at Cyborg's future inclusion in a film. Again, the next logical step would be to have Victor appear in this movie, perhaps during his surgery.