Damien Wayne (Son of Batman version), Starfire, Captain Atom, Cheetah, Black Canary, Power Girl, Constatine, Black Mantis, Prof. Zoom, Enchantress, Blue Beetle, Booster Gold, Tara or Jinx, Ra's Al Ghul, Penguin
I think we will probably get a guest fighter again like we did in the first game. Sub-Zero seems likely given how well Scorpion did in the first game. Maybe Max Rockatansky as a possible second guest (if they do more than one like MKX did) since he is a WB character and would be free for NRS to use.
For DC characters, I want to see Swamp Thing, Killer Croc, Blue Beetle and John Constantine. Black Manta is also a must too.
3 Wonder Woman
6 Green Lantern
9 Supergirl (well, not at all in her case but she's in so..)
10 Black Canary
11 Booster Gold
13 Blue Beetle
14 Doctor Fate
20 Harley Quinn
24 Ra's Al Ghul
25 Captain Cold
28 Gorilla Grodd (not a fan but he is in so..)
I can see Deadshot, Subzero + one more guest being two DLC choices, leaving another 5 options.
this roster would make me quite happy, a lot of "obscure" characters, half side from each is updated so the roster is fresh and there are also 8 females in there. personally I'd rather have someone else in Supergirl's slot cause she feels like a waste and would also be fine if one or two males were removed for some more girl power. e.g., I'd have no problem taking Captain Cold out and have Poison Ivy in or something
also, I am aware that Green Arrow or Cyborg for example are like 99% in but this is a wishlist ;P