Specifically, what was the planar arrangement in the "Known World" aka Mystara, where there were "Immortals", Draedens, Old Ones and whatnot? And any others of which I am unaware? How did the planar arrangement evolve, change and grow into the Great Wheel of Planescape?
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The cosmology in Frank Mentzer's Basic-Expert-Companion-Master-Immortal set version of D&D went like: Prime Plane, surrounded by four Elemental Planes (which existed in the Ethereal Plane, connected to the Prime Plane by wormholes), surrounded by the Ethereal Plane, which borders the Astral Plane, which contains an infinite number of Outer Planes which can be of any shape or size (but were all finite). Some Outer Planes were the size of solar systems, or bigger, while others were tiny. Some were one-dimensional, while others had more than four dimensions. Any dimensions beyond the fifth were inaccessable by gods or mortals because the Old Ones cut them off with the Vortex Dimension, which was the origin of vortex hounds (pets of the githyanki in 2nd and 3rd edition) and blackballs (3rd edition umbral blots). Who were servants of the Old Ones. The accessible dimensions were Height, Width, Depth, Hyperspace (used for teleporting), and the Nightmare Dimension (somewhat like a combination between the Plane of Shadow, the Plane of Dreams, and the Far Realm, if they were a bizarro parallel of the Prime mainly populated by chaotic good, diabolic-looking humanoids).
This cosmology didn't evolve into Planescape's. The Immortal Set came out in 1987, the same year as the AD&D Manual of the Planes, which used the Great Wheel. The two cosmologies existed simultaneously.
The Wrath of the Immortals boxed set, in 1993 or thereabouts, revised the BECMI OD&D cosmology somewhat, removing the six-dimensional Flatland physics and turning the concept of "dimensions" into something more like what 3rd edition "cosmologies" turned out to be. Besides the Dimension of Nightmares, WotI added the Dimension of Myth, which was a multiverse that resembled a magical version of Earth.
There weren't really any other cosmologies, just different versions of the Great Wheel. I wrote an article on the evolution of the Inner Planes and added it to the Wikipedia article on the subject: http://en.wikipedia.org/wiki/Inner_Plane - Jeff Swycaffer's Elemental Polyhedron ought to help satisfy your desire for exotic planar configurations.
However, note that in the 1st edition Player's Handbook (1978), there were a few differences. For one, it wasn't a wheel, it was a square. And the Outlands didn't exist yet - in the middle of the Outer Planar square was the Inner Planes.
Even earlier, in The Strategic Review magazine (the predecessor to Dragon), there was a chart that suggested that the natives of the Seven Heavens were called saints and the natives of the chaotic good plane (then called Elysium) were called godlings, and these were the good equivalents of devils and demons.
But anyway, it was mostly different drafts of the Great Wheel rather than anything radically different. Really early things sometimes used "Hell" and "Hades" interchangeably, or assumed a universe with God and Satan or Allah in it rather than the polytheistic fantasy multiverse we're used to. Note that Gary Gygax's own conception of the Great Wheel continued to evolve after he left TSR - his chart of the planes in Mythus Magic was a bit more baroque and complex than anything TSR ever produced. I don't own a copy of the book, so I can't give precise details, but the general idea was that he divided everything into "dimensions," so the Dimension of Probability mapped where a plane stood on the magic-technology axis, and another dimension mapped where it stood on the Astral-Ethereal spirit-matter axis, and so on.