Artificer: Either gone -or- functioning as "Indiana Jones" type ruin-explorers who have some understanding of the ways of pre-Olive Athas. Constructs focus on lesser materials like bone and wood.A lot of focus is put on the Psion when when all but 6 classes could warrant a variant rule, subclass, or new subsystem.
Artificer
Bard
Cleric
Druid
Paladin
Sorcerer
Warlock
Wizard
Only Barbarian, Druid. Fighter, Ranger, and Rogue could remain untouched if you are attempted to actually be faithful to any of the core ideas behind Dark Sun.
Then you get into the Cleansing Wars.
DarkSun isn't D&D set in a desert with some psionic stuff. Dark Sun changes a lot of the game and most of the lore doesn't make sense without those changes.
Bard: Revamped into a spell-less form with potions, poisons, and unguents.
Cleric: Really needs a "No or Light Armor" build shift and then either an Elemental Domain or a general understanding that any domain taken comes from the elements.
Druid: Probably cut, or significantly changed to essentially function as a "Specialized Elemental Cleric". Wild Shape gets big nerfs, or removed entirely.
Paladin: Oath of the Sorcerer King is the only option. That and Oathbreaker which are the heroic paladins for a change!
Sorcerer: Defiling and Preserving need to be put in.
Warlock: Either Pact of the Sorcerer King or some sort of "Ancient Entity" path that reaches across the eons of Athas. Probably Defiling by default.
Wizard: Defiling and Preserving needs to be put in.