For the sake of getting off the MX Nobody subject, I have an idea that isn't like those ideas. It could be a new form of MX, but what would be its purpose? Judging from the symbol looking like a cross between a Heartless and a Nobody, maybe the Emphatic Entity is like an "Anti-Nobody" meterial. I say the "Anti" part because although it may be made of a Body and Soul, the way it came into this world and its purpose and goal is different.
From what we know, MX's body was on the verge of death, correct? In order to escape death, MX transfered his Heart to Terra's Body and Soul, merging with his Heart to become Terranort. However, some assumed MX's Body and Soul died off, but what if it couldn't? I believe that the Emphatic Entity is a personification of whatever life MX had left, and it's still tied to his Heart. So, perhaps if Sora and co. kill the Emphatic Entity, MX would seperate from Terra, and they would only have to deal with a weakened MX to kill him off once and for all.