4. Booster Gold
Booster Gold was very subtly alluded to in the Man of Steel movie in a blink-and-you'd-miss it reference during an action scene. There was a very small sign saying "Blaze Comics" which, in the comic books, is the company that publishes the fictionalised version of the character's comic books, so the next logical step is surely to have the character himself appear somewhere in the next movie. Booster Gold is a flamboyant character who is also something of a show-off, so to have him in the background of a scene, mouthing off about how great he is to reporters, would be a perfectly suitable and acceptable way to have him cameo. He also appeared in Smallville quite successfully. He has no innate superpowers, but being from the future means he has an array of gadgets that grant him his abilities. As a fairly prominent member of the Justice League in the comics, it would be a cool hint of a potential team-up with the likes of Batman and Superman in a future DC Cinematic Universe movie.