This is a work in progress, so suggestions are quite welcome. Basically this is based on the fact that aboleths and other aberrations originally came from a realm before the standard D&D multiverse.
This is what I've come up with so far...
It is the realm before all realms.
It is nature of a different order.
It is the spawning place of the ancestors of aberrations.
Orsa is the primal beginnings before all beginnings. The realm that came before all that is known came to be. It is a place of not-liquids and semi-solid congealings of an ocean that could swallow the elemental plane of water and not notice. Entire solid realms though float within, universe-sized pockets surrounded by scabrous material of dried slime and bilestone miles thick. Threads of gel thousands of miles long spread and dissolve, all building towards something unknown. The entire plane is as a macroscopic being as thoughtless as a virus and just as adaptable. It is ultimately mindless unless one counts the id, of which it embodies at every level, making it both eternally simple as well as incomprehendingly complex.
Within all these places creatures grew from a soup of potential where biology and life had no limits. All could grow because it all was possible. Even the most improbable came to be; beings that could think. Thought could shape the ooze for it was eager to become. Hunger and strength is what rules, not to do battle, but to subdue and feast, to survive and absorb.
One might could say Orsa is naught but an eternally large physical consciousness, a great brain, and indeed its substance responds to the strongest will around. A number of the elder evils themselves are thought by some to have been the beginnings of true brains formed from the muck. Upon gaining knowledge of self however they also began to understand selfishness, and broke away to pursue their own goals. The living plane of Neth itself is theorized to be one such piece of the former realm.
* No Gravity:
Creatures of Orsa move by swimming or being absorbed by the stuff of the plane and congealing elsewhere. The latter acts as the Dimension Door spell, caster level equal to the creature's HD, and the being cannot bring another along.
* No Time:
Nothing ages in Orsa, they merely evolve, growing in complexity until they became something else, often becoming absorbed into the plane.
* Infinite Size
* No Alignment Trait:
The realm of Orsa has not yet conceived of morals or ethics, abstract ideas that have nothing to do with survival and adaption.
* No Magic:
Magic does not function in Orsa, the basic traits required not having yet evolved. Psionics however work perfectly normally as their rules are those of the mentality behind them, not what is beyond.
Orsa is where creatures not of the current multiverse originally came. Beings out of time. Holashner, Piscaethces, Shothotugg and so many others formed and gained sentience from the plane, horrible intelligences of deific power. Some rose from the muck as a fish learned to walk upon land, entering the Far Realm so as not to become one with Orsa. Only these truly survived what was to come. The spawning place evolving and coming together to explode outwards in a new way. A new place and new laws, a concept not known before. The multiverse that now exists, of heaven and hell, elements and shadow, man and elf, the gods themselves.
Many of the aberrations we know today, the aboleth and tsochar, gibbering mouthers and chuuls, are but the shriveled offspring of their progenitors, deformed by the current universe which does not follow the correct laws. The ancestors of these beings when they returned from the Far Realm found the most suitable places they could for their offspring, upon the new planes or upon the widely scattered worlds. They did not thrive, but they survived and remain so until this day.