I think the world axis cosmology can work, heavily altered, as suggested. The Elemental chaos works just fine as is. I like the idea of a feywild where the Formorians are winning, and a desert shadowfell... you thought having the sun beating down on you all day was bad... wait until thats gone too.
Having the Astral sea can be a bit of a problem - my inclination would be to blow it up... The astral sea is gone. Some of the denizens survived by making it to the world, or escaping to one of the other planes (so you can get your marut or devil etc when you need it) but across the board they are gone forever, bringing the gods with them.
Eladrin are mostly Formorian slaves, or escapees who have made it to the world, some have joined the bands of their kin, the elves, others are just trying to survive.
Dragonborn = dray. Pretty easy that.
Gnomes are extint... or thats what they want you to think. There are small groups of gnomes underground and they have become extremely xenophobic and keep practising the magics that destroyed the world. Others escaped back to the feywild only to be enslaved by the formorians.
Drow are twisted magical experiements on eladrin... see Eberron. Done by Formorians likely.
Genasi fit pretty well, they're elemental beings from the far too accessible elemental chaos.
Goliaths are Half-Giants, if they aren't I have no idea how to really dark-sunify them - they would just seem like tack-ons.
Shifters - aren't really nessesary, and likely won't be mentioned in the books.
Devas are the last vestiges of the old gods on Athas, a few angels made use of a ritual to become mortal in the last moments before the astral sea went kaboom.
Half-orcs - I'm not sure orcs and half-orcs are really nessesary to the world, and are likely going to be "thought to be extint, you are likely the last of your race"
Warforged - could be created by the sorceror kings, see the post a couple pages back with obsidian bodied 'forged. Rare.
Changelings - pretty much as is, works cool in any setting.
Kalashtar - not nessesary, psionic mutation if anything.
In any case, I doubt we'll see a 'Do not use these races or these classes in the setting' and it will be 'Other races don't exist unless you and DM can come up with an interesting story for them in this blasted world, and if you include them, they are likely to be extremely rare.'