I would like to see a level of setting detail similar to the what Eberron got, but with some kind of novel take.
Jump the setting forward to 50 years after 2e's Beyond the Prism Pentad:
All the books' main characters except maybe Sadira are dead from old age (because she makes an interesting alternate sorcerer queen). Dregoth laid to waste and occupied Raam; his dray are now everywhere. Balic is greatly diminished from internal conflict, but Andropinis is somehow influencing it from the black. Tectuctitlay's "son" Atzetuk is a powerful psychic who rules Draj. There's lots of religious strife between Lalali-Puy's proseletyzers, Dregoth's cultists, Atzetuk's moon priests, and others rightly suspicious of them.
Jump the setting hundreds of years back to the post-cleansing wars:
Athas is newly desolated. It's a bit greener because it hasn't been defiled as long, but there's a much greater sense of what has been lost--shorelines replaced with salt flats, ruined port cities, peoples with traditions held over from the good times, a memory of the genocides. Borys is still nuts, Dregoth hasn't been betrayed and undeadified yet, Sielba still rules in Yaramuke, Kalidnay hasn't been sucked into Raveloft yet. The sorcer kings are still kind of cooperating, but a cold war is beginning to settle in.
Move the setting out of the Tyr region to some other part of the tablelands--one that maintains similar themes, i.e. terrible scarcity and immortal jerkwads destroying the planet, but with different personalities and adventure locations.
Something that is not the status quo from the original box set... we got that 2 times already.
In any case, the deep lore abut Rajaat and the blue age and so on and so forth needs to be vague and hidden. It's background fluff that ordinary people don't know anything about outside of rumors.
I’m still curious about what side setting agnostic mechanic or subsystem they could add to the book, as an additional selling feature.
Maybe something related to multiple characters, like what 2nd Edition did. Advice on how to manage it and when you can switch and how treasure/ magic is handled.
Mechanics for a character tree would be great, emphasizing Dark Sun's deadliness and enabling other settings to borrow some of that same deadliness. That is something I would like to see.
I would also like the strong restrictions on races and classes that others have mentioned.
Where defiling/preserving is concerned, I'd prefer a subsystem that all arcane classes can use. To make defiling tempting for players, I think the mechanics should be simple and relatively powerful but acquisition of long term defiling stigmas/penalties should be random.