I miss Dark Sun and it is a pity it was "cancelled" because the current standars are different. I have thought about some spiritual succesor but I see if we want total freedom to can add some new crunch element, then we should start from zero. 

The voice of the deities can't be listened, but, according the official sources, by the sorcerer-kings. This cult is the only allowed religion. 

The main region was devastated by the defiler magic. There are green zones beyond the Tablelands but arcane spellcasters aren't wellcome. Some plants can survive the defiler magic but then these are "tainted" and too toxic to can be used for wood or food. 

Living constructs (warforged, autognomes, shardminds, glitchlings) are allowed PC species but with one condiction: There is an elemental "fungus" afecting constructs and then water and "food" is necessary to "resist". Some constructs "wear" a living simbiont to hide their construct traits, but these need food and water. 

There is a secret demiplane what a sea full of life, but only haflings are allowed. The ruler can "sell" the water, Some times there are flood of "brown tide", tainted water being expeled. This can be used to irrigate crops, but not drinkable by animals or humanoids.

 There are elans, mainly among the elite of the city-states, as a reward by the sorcerer-kings. There are also "psionic" species: maenads, xephs, dromites and synads. A lot of times because reincarnation ritual is too unpredictable. In far zones, other continets beyond silt seas, there are also other species like hadozees, plasmoids, rastipedes, fraals (little grayskin aliens), shesheyan, reptilians t'sa and even a biopunk version of aleerin/mechanus.  

The sorcerer-kings are mainly like infernal (howling , Styx, pyroclastic, rust, Tarterian, gloom, chole, and deathmask)  half-dragons, not cromatic or metalic. There are "lower" dragonborns. 

Magic can be used to create food and water, but this is too "easy" to be detected by the templars. 

Sorcerers who cast divine or primal magic instead arcane are possible. 

Some alchemic tricks allow effects like arcane magic but without defiler effect (but some polluting gas).

There are gladiator games, but lots of times the deaths are fakes thanks illusory magic.

In other zones of the "wildspace" there is a Gith empire. These aren't like githyankis but more with dracotouched traits. Their main rivals for the control of the wildspace is a "thoon cult" by formians. (Thoons from MMV in 3rd Ed). 

Not all sentient souls become undead spirits but they are reincarnated as elementals. Some sinner souls become fire elementals, and they are used as "fuel" for magic items. Water elementals are too coveted for their healing gifts. 

There are faeries, but without buttefly wings, but like wasps or moths, and they are relatively unfriendly.

A cleasing war started, but most of population could evacuate toward other zones of the wildspace or the "land-within-the-wind" (Feywild). This stopped when the first formian waved arrived. The invasion failed thanks "drows" (transgenic elves created and controlled by the shadow spiders) helped shadar-kai to start a rebellion against the formians.

There are werewermins....and saurian shifters.

There aren't clerics of Vecna or Tharizdum (the chained good or elder elemental eye) but "ardents" (psionic instead divine magic). 

There are some shamanic cults of kaiju-like creatures, and now someones start to become relatively sentients. 

There are gem dragons, but these would rather to be hidden because they fear the sorcerer-kings. 

Theorically the spinewyrms aren't sentients, but there are reports of these behaving like founders and leaders or new rogue factions. Maybe some spellcasters discovered the way to hunt and steal their power.  

WotC is allowed to use these ideas, I ask the idea was found in an internet forum. 

Here I have got an idea about how to create a Dark Sun spin-off allowing the style you wanted, keeping the original essence or with enough freedom to add new elements. 

Next to the Athaspace there is a second wildspace. When the cleasing war started this became the home of refugees. Rajaat allowed this evacuation because this facilitated his own plans. Of course the defiler spellcasters also arrived and started to cause serious troubles. And as if this was not enough the wildspace also suffered attempts of invasion by githyankis, formians and a rougue faction from the vodoni empire. The Feywild and Shadowfell also were affected, and when some region was too "tainted" then there was a planar shaterring and the "broken pieces" become a demiplane or astral domain, something like the spirit realm fom Kara-Tur. 

After a long time and a lot of pain and sacrifices finnaly the sorcerer-kings of this wildspace were defeated, or at least most of them.