Zatanna, Raven, and Poison Ivy are the ones I want the most and the announcement of any or all of them would make this game an instant buy for me.
Huntress, Black Canary, Cheetah, Starfire, Terra, and even Star Saphire would be neat choices too. I also wouldn't mind Supergirl/Power Girl or one of the Batgirls as long as they play differently than their male counterparts. I would be excited if they sort of threw us a curveball and added a lesser known female like Vixen, Killer Frost, Rocket, Mary Marvel, Bumblebee, etc. I mean, before MvC3 came out, when characters like Taskmaster and X-23 were announced I remember people complaining about how obscure they were, but after getting a chance to play them they became fan favorites in the game. The same thing could happen with a character like Vixen or Killer Frost.
Vixen (if she gets in ill be satisfied honestly ) Poison Ivy Catwoman Hawk Girl Star Girl Zatanna
Seriously though has anyone else ever read up on Vixen? She's done some pretty impressive stuff. Id definitely rank her in the top tier of the JL. It'd be interesting to see her animal powers translated into a game like this. What do you think her chances are?
Now that I think of it, there are several lesser known DC females that could fit right into a Mortal Kombat-style game, like Scandal Savage, Katana, and Big Barda (or most of the Female Furies actually). I remember in the DC animated Justice League cartoon, there was this blindfolded female sword user that reminded me a lot of Kenshi from the Mortal Kombat series. Her outfit even looked very similar. I forget her name, but she might fit in too.
However, it is incredibly unlikely that any of these characters would be popular enough to make it into even any video game, much less this one. Its nice to dream though.