B/X Known World
The OP is a good list, but more than half of them are already how default Dark Sun works. Most of what's quoted below isn't a change at all.
That is the default of Athas outside of the city-states. Lots of tribes live out in the wastes. This is established in the fiction and the game products.2) It'd be Points of Light.
Small desert encampments of good people trying hard to survive in a terrible world. Good people in terrible situations in various city-states. Yes, the world is largely destroyed, but even the Sorcerer Kings couldn't wipe out -everyone- on the planet.
That has been the default assumption since the original boxed set was published. Kalak died in the first novel and was killed off in the first non-core box product released for Dark Sun. But they generally advance the timeline to have Tithian as the king of Tyr. But there's still all kinds of chaos in that city-state.4) Kalak is dead.
Tyr the Free City would not be devoid of problems. But Kalak would be in the ground having been killed several days before the start of the campaign setting timeline. Yes. Days. Meaning it's still Chaos in Tyr. Wealthy merchants vying for control of the city through brute force being killed in their homes by revolutionaries.
That's exactly how it works in the fiction. You can cast slow and carefully to preserve nature or cast fast and recklessly to defile nature.5) Defiling is not the basic nature of Magic, but a specific use.
Defiling isn't how spellcasting works, but is a tempting and constantly available power boost. Withering a 5ft patch of grass adds +4 damage to any spell. Killing a tree adds +30. Things of that nature. HP taken from party members is a straight 1 to 1 damage boost, but reduces your ally's hit points by the same value until restored, or 1hp per long rest.
The preservers in the fiction and game products do that with knowledge of history. The elemental clerics and druids are more focused on the restoration of nature.8) Preservers are trying to save the world.
Largely outside of cities, the Preservers are a group of people who are doing all that they can to preserve what remains of the world and restore some measure of what was lost. They have hope. Maybe it'll work. Maybe it won't. But they're trying to find pockets of life and plants and good soil in the world... and collect them. Bring the soil and the seeds and the life together to try and slowly restore the world.
There are several game products that expand the setting beyond the Tablelands.9) It's not just the Tablelands.
The Tablelands are Borys' place. His territory, with the heart of his "Empire" in the Silt Sea. Beyond it are things like the Crimson Wastes and the Stone Jungle, a place the defilers destroyed but the petrified trees didn't collapse like they largely did in the Tablelands. Yes, there are people in the Stone Jungle and the Crimson Wastes. No. They're not more friendly than the people of the Tablelands. They may or may not have Sorcerer Kings of their own. No one knows. Their languages aren't known in the Tablelands.
There are still gods in Athas. They just don't have active, living worshipers. In the first novel some of the characters find a long-abandoned church under Tyr that is guarded by spirits of dead clerics and paladins of a long-forgotten god. But turning them into an active force to antagonize the sorcerer-kings would be a neat idea.10) There are still Gods.
Oh, this one would be controversial... but yeah. In addition to the Elementals, there would be gods in the world, yet. But they'd be practically titans and forgotten gods (a5e Monstrous Menagerie). Creatures you could theoretically kill that exist on a mortal level. But they exist and could grant some measure of power to a hundred followers or so... but as the world dies, so too do they. And they are greatly weakened, and occasionally wrathful. They fear the Sorcerer Kings... who have stolen much of their power. And may yet take the last of it. They tend to masquerade as mortals and hide, acting almost exclusively through their agents.
That's exactly how sorcerer-kings work in baseline Dark Sun. Always have. The templars are the clerics of the sorcerer-kings, in 2E parlance, the priests. Complete with cleric class, spell progression, etc.11) The Sorcerer Kings are also Gods, basically.
They're not full power external to human experience god-beings that drift across the cosmos creating things... But they can directly grant power to their servants like a god would. And have more servants than the "Old Gods". Also killing one of the Old Gods in a ritual could be a path to power for, say, Nibenay or one of the others.
There is still rain on Athas. It's just rare. You don't get all the green on the maps without rain and/or moisture of some kind. Rain is mentioned several times in the first novel and in various game products. Though the Sea of Silt is not an ocean. That would be an interesting if thematically odd choice.12) There would be rain.
Fitful, hot, and unsatisfying. Occasionally toxic. There would still be rain on Athas... but the thirsting sands consume it and drag it away... The Sea of Silt is actually a -sea-. With some water intermingled with the Silt. And Silt Sea Raiders do all they can to filter out another mouthful of greywater from what moisture remains. After rains cross the Silt Sea, there are -days- of ease and leisure for the Silt Sea Raiders... but once the water slips down past the first 20-30 feet of silt, it's practically impossible to get at. Never enough rain to slake the thirsty masses in the City-States... but enough to let the rovers barely survive.