Heartless, Nobodies, Unbirths?

[Tavrobel]

likely that would be Zexion

Why him? Not to insult your idea or anything, but I fail to have seen anything from the games that would suggest that Zexion is any different from the other Organization's first six members. He might have been the youngest (and therefore, the most idealistic), but at his core, he was just like Vexen and Lexaeus, or Luxord and Xaldin for that matter.

The only other way that I think you could mean is that his untimely demise at the hands of the clone Riku could affect his respawn? If so, then it could possibly stand that Lexaeus would return as well (even though it was the real Riku in this instance). If we wanted to take it even further, Vexen or Axel could show up again; the others were defeated by Sora.

hm....what i'm thinking is that If Heartless are Entities of the dark and it was said the Nobodies were to put it in balance, If a Heartless were created then a nobody was formed. but what if there has to be a light counter part to the heartless, Being Unbirths. This may have already been repeated once before in a different use of wording.

You are confusing your causalities. Nobodies form because a Heartless is formed, not the other way around. Just because a Heartless forms does not guarantee that a Nobody would form. There's no balance here, it's just a shell that the person left behind when discarding their Heart.

Unbirths have yet to be explained in full, but any guess is as good as anything else. Sounds really obvious, though.