Sub-Zero (neither Bi-Han or Kuai-Han Liang) never had a significant other and never had any children, so there is no way this could be a Cryomancer or a descendant of Kuai Liang. He may have escaped the Netherrealm. Perhaps Quan Chi lost his mental hold on him. I've heard a rumour that Sub-Zero got reanimated and became a similar spectre as Scorpion, only he gets to keep his flesh. Notice his Lin Kuei symbol? His return might have something to do with the Lin Kuei, as they are allies of Outworld and Netherrealm. Ed Boon said that Kitana will also return. I don't know how she could return but I hope it will be explained.