I'm going to support this theory as credible.
Initially it sounds almost idiotic, but the more I think about it, the more it makes sense.
Let's look at the names of the enemies so far and see what they have for connotations.
Heartless and Nobodies. Both have literal definitions for the words that oppose their concept.
Heartless DO have hearts. The word is used to describe how they're cruel, emotionless creatures of darkness.
Nobodies DO have bodies. The word is used to describe how they're essentially non-existent.
So both names contain a root word (Heart, body) with a negative suffix/prefix (less, No), respectively.
Apply this to the Unbirth.
Root word: Birth
Prefix: Un
The negative prefix denotes the opposite of the root word. In other words, we're looking for an antonym of birth. The antonym for birth is death.
Now, some may argue against this, but that's just silly. Death is the antonym for birth, I don't know how else to say it, go check a dictionary. Some words can hold antonyms in common despite not meaning the exact same thing (birth and life). You are alive (life), you are no longer alive (death). You come into life (birth), you go out of life (death).
So, with that in mind, let's move on.
In the same manner as Heartless and Nobodies, Unbirth DO have births, but the word "unbirth" is used to describe them in some way.
How would having no soul, only a heart and a body, constitute as being "unborn"?
Well, first and foremost, to those naysayers that deny this on the grounds of the Secret Ansem Reports, I say poo-poo. Xehanort's Ansem Reports have been shown to have faulty reasoning, so we cannot assume that AtW is faultless. But, actually, this doesn't even have to contradict the Secret Ansem Reports.
Who said they necessarily have to lose souls? When a soul is lost, the being gives way to death. But what if a being could conceivably be born without a soul in the first place?
How can said being have "life," you ask? Well, as Nomura has said, a heart is proof of life. You have a heart, you're considered alive. Despite the fact that a soul is necessary to maintain that life, could it be possible that something in the initial condition of a soul-less being allows it to be "alive"? Think about it. If Heartless (only a heart) can be sentient, why couldn't an Unbirth (heart and body)? If anything, a "living" heart and body sounds more credible than just a "living" heart.
You might be saying, "Hey, then that means an Unbirth is basically a Heartless with a body." Well, more or less, yes. But I think it's a great concept. The Heartless, comprised of a heart, do not need anything else. The heart is the source of all power. But this is a chance to have a being with a heart lack, and possibly desire a soul. It would be interesting to compare them to the Nobodies. Plus, the Unbirth are the origin of the Heartless/Nobodies, and this theory makes sense of that. An Unbirth is one step before a Heartless, and acts as an intermediate being between Heartless and Nobodies.
Heart--Heart and Body--Body and Soul
You could technically consider the Unbirth to be more like the Undead, but I highly doubt they'd be portrayed as zombie-esque in KH.
So let's go back to the whole name game.
Heartless DO have hearts. The word is used to describe how they're cruel, emotionless creatures of darkness.
Nobodies DO have bodies. The word is used to describe how they're essentially non-existent.
Unbirths DO have births (their creation). The word is used to describe how, despite being "sentient," they aren't truly alive, not by any conventional meaning.
One of the reasons why I think this is a great idea is that the game will get to divulge just exactly how the soul works. If Chain of Memories/Days can emphasize memories, I don't see why Birth by Sleep wouldn't be allowed to focus on another aspect of the being besides the heart. And it's just a simple, straightforward concept in the face of many others which are frankly getting to complicated (funny coming from me).
Now, how they come into existence is another story, but it's given me something to think about.
So, yes, believe it or not, props.