How does the name Unbirth have meaning? (Beyond being born by being comatose)
First I'm gonna be blunt...Unbirth's might in fact be BwHoPL(Beings with Hearts of Pure Light) or something similar to it.
Now here's the confusing part because it get's into babies...yes, babies, and not the "in the womb" kind either. Babies when they are born, especially in the real world, are totally and completely innocent, and (sorry to bring religion into this, but I think it's somewhat necessary) if they were to die as a baby, they would be believed by some to go directly to heaven because they have not committed any sin or had a chance to even commit to god....in KH terms the idea would be that they have no darkness in their hearts, full of light, but not forever of course. They can be corrupted as everyone is at least a little bit, aside from the 7 Princesses of Heart, who according to this would be beings immune to the corruption of darkness and thus remain with hearts full of light. When babies are born, they not only are totally and completely innocent, but do not have any memories, sense of self, those three are developed as life goes on. Unbirths would of course be similar to this, except babies are naturally this way.
Here's where it gets more into meaning of the word...by having a being with fully developed memories, sense of self, and purpose, suddenly lose them, their hearts are unnaturally reverted to the state of an infant having just been born and are now, in a way, re-born, kind of ironic with the name no? Except these beings are not meant to be in that condition and thus the light they have is now corrupt because any darkness that would have been in their hearts left with the memories and all that. Thus creating a sort of mutant for lack of a better term giving meaning to the word Unbirth in a similar fashion to that of a Heartless and Nobody.
Last major obstacle and one I admittedly can't fully defeat at this time...Why are they extinct?
In order to even begin to answer this I have to explain another crazy idea...the Kingdom Hearts Master Xehanort summoned in the secret trailer was the Kingdom Hearts of memories. Like a giant heart, it gathers the contents of the hearts of those who are now Unbirths in itself. I believe that this Kingdom Hearts may not even exist anymore due to events that took place in the BBS game and it is now impossible to become an Unbirth. Why and how this all happened I'm sorry to say I don't know or can't think of a decent explanation at this time that woudn't require vast knowledge of BBS. A thought I had about this would be that MX was searching for it in order to harness the powers of people's memories...or perhaps retrieve those of a specific individual. I also have ideas concerning Ven and DS becoming, pertaining to Ven, or already being, directed to DS, Unbirths.
Phew, sorry that was so long. I'm glad I got that all out, and believe it or not, I probably forgot to mention something...hopefully someone will make a counter argument that will jog my memory. If you read this far I thank you for your time, hopefully this made sense to you. I am looking forward to any debates that may come out of this.