There are certain traits God must have. He must be self-necessary, must be able to exist independant of any other entity, and must be eternal (which subsequently does not allow for change). If He is not those, He cannot be the original being, or non-being.
The reason why God must be eternal is that to be self-necessary, you can not have had a beginning, for all that begins is caused by a prior actuallity, and therefore is not self-necessary.
To be eternal, you cannot have change. As soon as you have change, you have a created a reference point, which means God exists in an event-by-event sequence (finite time), meaning that He cannot be self-necessary, for He would be dependant on time.
This is why I feel only God could be the original being. Because of the change, and event-by-event sequence evident in the world, we know that time is finite. Therefore, it could not be a natural entity, for all natural entities are finite.
Basically, the original being must have been eternal, yet only a self-necessary entity could be eternal, yet natural entities are not self-necessary. For an entity to be self necessary, it is no longer natural, it is beyond natural, therefore contradicting the essence of natural entities. Obviously, there can only be one self-necessary entity.
That doesn't mean there can't be other 'supernatural' entities, such as angels etc., just that if they exist, they would not be self-necessary and not eternal. What would make them supernatural is that they would be able to transcend the universe and it's laws, but that's another story.
Hope it all made sense.