Having accepted that Zod and Scorpion are forthcoming, I now focus on wishing for a second round of four DLC characters.
Taking into consideration that WB may want to loop in a cross-promotion (e.g. Man of Steel Zod DLC), the only two major promotions would be Infinite Crisis and Batman: Arkham Origins. The two characters (announced thus far) from Infinite Crisis that isn't in Injustice are Zatanna and Poison Ivy; from Origins: Black Mask (and Penguin if we are specifically talking about playable). I don't see Black Mask being an option for the game.
If I had it my way: Zatanna & Martian Manhunter / Black Manta & Despero would be the most rounded selection for play styles.
I'd like more grapple-heavy power characters. I wouldn't mind Mongul, Darkseid, Black Beetle (gadget/grappler mix), King Shark or Gorilla Grodd taking Despero's slot either. I'm not a fan of Bane, so that leaves me between Grundy or Doomsday. I've been using Doomsday the most of the three.
Sure, Darkseid would be great, but I don't see him making the game. There have been no hints to my knowledge of him making it in besides his cameo in a non-playable role. Red Hood and Poison Ivy both have a better chance than Darkseid for playable. Black Manta & Despero don't have great chances either, but I'd rather play as those two more than Darkseid, especially since they both aren't from the Superman and Batman rogues gallery. The villains are the hardest to gauge. When Killer Frost and Ares make it into the game from left field then anything goes!