Lol, I was wondering when people would actually start figuring out that they don't just need to fall in line with one of the proposed theories. We're men, not sheep...

My theory: There have always been Hollows, as it seems to be the final form of human sin and decay of the soul, a trait that has been present ever since humans were created.

However, I often wonder when the Shinigami came to be, as it does not seem that humans dedicated to fighting the Hollows would be a natural creation. It is plausible that some people, after death, wanted nothing more than to destroy the evil Hollows and dedicated their after lives in order to sharpen their skills.

These humans were aided by another kind of spirit, which would become their weapons against the Hollows. I am speaking about Zanpaktaous/Soul Slayers, which all seem to have existences of their own and are notably NOT human.

As for the actual origin of Soul Society, the human world and Hueco Mundo I believe they were created as separate systems in order to preserve the balance of souls. Hollows had to be confined to Hueco Mundo or they would probably bring great disaster to the living, perhaps even destroy all life. Soul Society was created as a place for the deceased souls to go, rather than have them all milling around the human world, which would also be quite a disturbance... The ultimate goal of the creation of these separate realms was to facilitate the human world, which is the crux and actual center of "life". While souls may go to SS or Hueco Mundo, there needs to be a place for new life to be created and raised, and this is the human world.