Why Shinnok? Because they showed him at the very end of the story? Did you listen to his conversation with Quan Chi? It didnt sound like he was in the world it sounded like he was in another place. How would they explain him being in the DLC when he is still
in that other place? Time travel? That already worked with Raiden lets not press our luck with Shinnok. Besides Shinnok is left better for an expansion or a sequel not DLC.