MAJOR SPOILER ALERT
Spoiler:Could it be Mileena's flesh pit costume?!
Spoiler:NO, She is nekkid with her flesh pit costume, like in her Vignette!
/Hope
Oh, just heard a good theory, that might simply be an unrevealed character that they simply fleshed out so that we can't see who it is, but who? Maybe the red lady...
No, I think Shang's X-ray involves turning temporarily the opponent into a skinless zombie-like by stripping him/her of his soul or performing some sorcery act; remember one of the Lupinko's leaks in which he said Quan Chi's x-ray will turn the opponent into a zombie? I think this is the case and maybe he mixed things up and said Quan Chi instead of Shang Tsung, or maybe they have similar x-rays.
Think of it, it makes sense.