The whole meme of the 4e planes being based somehow on "literary precedent" and the Great Wheel planes not, needs to die. Specific to what you said above, the classic D&D elemental planes (and the ethereal plane between them and the Material) are based on the classical pre-Socratic greek elements, while the elemental chaos is IMO a confused and cliche pastiche of Limbo and the Elemental planes of the Great Wheel without any specific literary inspiration.
Actually, you're dead wrong on this.
In the cosmology of Milton's
Paradise Lost, Heaven, Earth, and Hell are all floating within a vast realm of matter without form or order called Chaos. This is supposedly the realm of pure physicality that God retracted from in order to create Heaven. The ruler of this realm, Chaos, the "Anarch of Old" is presented as a ruler who keeps a court filled filled with powerful beings (one name among his courtiers is Orcus), and exists primarily to make Lucifer realize how minor and trivial a thorn in the side of God he really is. It is also described that Chaos is a tumultuous realm where Lucifer is battered by unpredictable flames, ice, lightning, and earth as he attempts to pass through it.
Sounds like the Elemental Chaos to me. It may not be a widespread cosmology, but it is nonetheless literary precedent.
Edit: Also, I may as well mention that Paradise Lost was the origin of the names Limbo and Pandemonium for planes of the Great Wheel (though both are different from how they appear in Milton's work), so you can't very well reject it as a possible bit of inspiration for D&D.
Also, this quote from an
article on Wikipedia might be relevant: "Ovid (1st century BC), in his Metamorphoses, described Chaos as 'a rude and undeveloped mass, that nothing made except a ponderous weight; and all discordant elements confused, were there congested in a shapeless heap.'"
There are vague mythological notions of reality rising up out of an original chaos, and that's what I used for Pathfinder's CN Maelstrom (specifically riffing off of the Ogdoad of Hermopolis from Egyptian mythology), but there's nothing specific in the 4e cosmology that derives beyond the most vague notion from any of that, versus the use of the classical elements for the elemental planes of the Great Wheel.
Honestly, I don't think the idea that, just because you want to use the four elements that they have to be represented in something so bizarre as four pure planes. I mean, the idea of
wells of elements show up (like in Norse mythology), but those always lead to the ideas of those elements spreading out and mixing in order to create something (as in how the flames of Muspelheim and ice of Nifleheim spread and mixed within the vast void of Ginnungagap). The idea of giant realms purely dedicated to a set of elements is just... strange.
I have no idea why pure planes are at all necessary, especially since they're about the only place in D&D where the four classical elements are actually relevant. It is not like the game's fluff and mechanics are exactly in love with them (like some games I've seen). You could change the list of main cosmological elements and it wouldn't impact the mechanics of the game or any of its major settings in the slightest, or just drop the elemental planes entirely without issue. Also, they're boring as heck, so any alternative that isn't boring helps a lot (and yes, I know you've run a campaign that made adventures out of places like the Deep Ethereal, but that doesn't change my opinion in the slightest).
