Robin is a lock for this game as there is no way a tie in comic for the game would showcase cast not in the game itself. However, I'd say that since Nightwing was given a staff which he has never had for no reason that its pretty likely he is just an alt for Nightwing in costumes otherwise they took his gimmick and gave it away for no discernable reason which is HIGHLY unlikely. Its a smart way to get more cast into the story without having to honest to god rig, animate, and balance a ton more characters.
I'm betting the Green Lantern slot is shared by Hal, Kyle, John, Guy and possibly Baz with either Atrocitus or Sinestro being used as another character for them to fight taking up a seperate slot on the roster. The main 4-5 guys from Earth that are Lanterns will likely share one slot and be alt outfits for each other though IMO. Just smart marketing to appeal to the fans and let them all play as who they want.
BTW enough with the Batman and Friends comments please. We have 24 slots minimal and we only got around half even hinted at so far and Batman is just the easiest market to use to draw initial hype and numbers to the game in the form of pre-orders. Just give it time. Boon's already promising the next several reveals won't likely be Batman related and states that there are PLENTY of non-Batman cast coming. Just remember that his comic has the most casual market saturation and its money to promote them early and often to draw those folks in.