Really want Poison Ivy and Two Face but that may be too much of Batmans world in the game.
Yeah, as much as I want Batman rogues to show up, having characters such as Killer Frost and Ares in the game begs for more under-utilized characters to be given a shot.
If the DLC is absolutely going the route of 2 heroes and 2 villains, then Lobo and Etrigan can easily apply due to their anti-hero stance, but I wouldn't want them both if there's only 4 characters coming. The list of female villains in DC isn't extensive -- the most well-known ones are from Batman and I'd hate to see Injustice become a Batman-dominated game. There are already 6 Batman characters in the game, not including Deathstroke.