4. Mongul
With most people demanding, and indeed expecting, the extra-terrestrial threat in the
Man of Steel universe to come from Darkseid, it might be a satisfying alternative to surprise them with an appearance by an equally formidable alien being - Mongul. Mongul first appeared on panel when he was exiled in to space by his own people and ended up on Earth. He kidnapped some people close to Superman, including Lois Lane, and wouldn't return them safely until Superman obtained and brought to him a key which would activate an artificial planet called Warworld, which was created to be the ultimate weapon. With such a (relatively) simple backstory, it wouldn't be difficult to introduce Mongul to the live action DC universe and with modern CGI he could look really cool. He could be the key to linking the Superman movies to the Green Lantern movies, perhaps by getting his hands on a Sinestro Corps ring - like he does in the comics - maybe in a post-credits scene, having been banished to space by the man of steel himself.