DiZ could easily be seeking Sora (Who is the Defender of Light), that's why he is contacting Sora's friends.
Sora WAS a Heartless, but became a being again. However...
"You Are Incomplete..."
This strongly implies that he was NOT made completely whole again, so BHK, Sora etc., could have pieces of heart, soul, body, but Sora would technically be a Heartless, since his heart is the main sustenance of his being, BHK Nobody, etc. Remember, BHK is the "other side" of Sora's heart, or at least this is STRONGLY implied.
And besides, we've all seen that Sora had ****loads of heart, and Kairi had even more (Princess OF HEART). They could easily have strong enough hearts to operate the keyblade, still have being, etc. without being complete.
2: The Disney Villians are back, they could be the ones behind the Heartless: We only see cloaked members being nice to Nobodies, Heartless are slain (DD, ASAS).
3: Sora is the defender of light, not made of light. Many times through the KH series, especially RIGHT AT THE BEGINNING, it is said that you CANNOT have light without dark. Especially since hearts seem to come from darkness (bodies form twilight, souls from light).
To make this possibly easier: Hearts, like with certain japanese ghosts, could be the emotions or drive/purpose, the place or source of our base, nonenlightened actions, or possibly just our non-rational actions. Thus, darkness.
The body, as a manifestation of our actions and purposes, and as a vessel, is neither rational nor non-rational. It is in-between, a power seperate: Twilight.
The soul/mind, the power of rational, enlightened though, allows us to think beyond the now, to plan, have free will, etc. Thus, Light.
To be a being, you must have all 3 of these powers united in harmony. Sora might not be, he could just be strong enough in opne of these to compensate.
PS: Please tell me if you think this is utter twat or not.