The planes of men are supposedly where all five elements are in balance. It then mentioned the elemental planes, but I never heard of a true plane of man. The astral plane is a plane of energy so it could just be the elements mixing together, but lacking any materialization of individual aspects and just acts as the primordial plane of chaos from which order arose that you often hear of in most mythos. That leaves us lacking an elemental plane of man, and since they are so the element is so little understood the abyss could be a natural plane of men and the divine realms could be imitations of it. It was mentioned that the Abyss was a plane on a near infinite scale, but no natural plane is normally that large, except the elemental planes, which are all listed in singular so far as in one plane of fire, water, air, and earth.
This is all mere conjecture at this point. The abyss could simply be a breeding grounds for invisible super rabbits, or a realm meant to hold an Eldritch Abomination beyond mortal comprehension. Remember nothing is truly valid until the ancient, and not senile, stoned, schizophrenic demon confirms thing over a pipe.