Not a bad theory, but I have problems with it. Mainly, if Unborn were the hearts that have yet to be born, then they simply wouldn't exist.
I believe that the Unborn are tied to the soul, as the Heartless are tied to the heart and the Nobodies to the body. I quote Ansem: "The three elements that compose life...the heart, the soul, the body."(Taken from: KHUltimania). When the heart leaves the body, a heartless and nobody are created. When the soul leaves the body, life dies... but what about souls that never had bodies? That is what I believe the Unborn to be. They are still technically "unborn" as they have not been incarnated as a human yet, though they are still very much in existence.
I believe that the Unborn are tied to the soul, as the Heartless are tied to the heart and the Nobodies to the body. I quote Ansem: "The three elements that compose life...the heart, the soul, the body."(Taken from: KHUltimania). When the heart leaves the body, a heartless and nobody are created. When the soul leaves the body, life dies... but what about souls that never had bodies? That is what I believe the Unborn to be. They are still technically "unborn" as they have not been incarnated as a human yet, though they are still very much in existence.