I want Ares to come back. I know he has one of the lowest chances of being a returning character but I'd love to see him come back with a revamped moveset where he uses more weapons. Also, as a Doomsday main, I'd like to see him return but with a new trait or some changes to his current one. It was kind of irritating how Superman and Bane's traits made Doomsday's useless and how easy it was to beat. I'd also like interactables to return, but be blockable this time around. Stage transitions are another must, they are part of what makes Injustice unique. In fact, I'd love to see interactables and transitions incoporated into something like a stage fatality/brutality. I know DC won't let its characters get their limbs ripped off like MKX, but maybe do something like Killer Instinct where they just get turned to stone or punched into a black hole or something like that. Traits should and will obviously come back.
As for new characters, Doctor Fate, Black Manta, Darkseid, Swamp Thing, Atrocitus, Black Canary, Jonah Hex, Clayface, Poison Ivy, and Ra's Al Ghul would all be cool to see.
