What i'm most looking forward too are detailed skins that can turn one character visually and animation-wise into another without it being a clone or separate character.
Like the human cyborgs in MK9, Cyborg Superman, Batman Beyond, John Stewart, and I think a few others that are escaping me right now.
Like a intricate Kintaro skin for Goro, or a Frost skin for Subzero, etc. complete with different voices, intros, and altered animations.
Hell they could even make a Noob costume for Subzero by just changing the ice effects to that sticky shadow goop(sticking opponents instead of freezing them, sliding on it instead of ice, throwing a shadow clone instead of an ice clone, etc).