I beleive that since Lui Kang is back and that we know he will be in Zombie form. well i think for the first part of the game anyway. Also I beleive Shang Tsung will be playable in MKD.

Check this theory out.

Lui Kang rises from the grave. A soul less body. A Zombie searching for Tsung. Eventually in the game they fight.

When Lui Kang beats Tsung, he will focre the sorcercer to release his soul, and Lui Kangs Zombie form will change and he will be transformed into a ghost form. (also this means that the ghost form will now be unlockable or just a new costume, anyway you can choose which want from here on)

By ghost I assume he still will be solid, just the fact he is dead but now dead with a soul.