Good point about subclasses. If the core setting nixes classes, how detailed do you get with respect to subclasses? Can a player play an Eldritch Knight? An Arcane trickster? Are there special rules for defiling if you’re not a full caster? What about a divine soul sorcerer? It’s an arcane caster but very cleric-adjacent.
Do Rune Knight PCs defile when they cast? How about Phantom Rogues? What about Totem Barbarians when they cast Beast Sense, Speak with Animals or Commune with Nature? Even if certain classes are banned, are subclasses permitted? It seems that a Circle of Land (Desert) druid would be appropriate for Athas.
Yeah. Subclasses are sometimes more important than the class when deciding on setting flavor.
Any Arcane magic, including Trickster and Eldritch, defiles.
Defiling is the magical equivalent of nuclear fission. For defiling rules, I prefer something visceral and concrete, such as requiring living plants within a radius, which the Defiler must personally destroy in order to catalyze the annihilation of Elemental Water. It is this Elemental annihilation that releases the arcane energy. Note, any and every arcanist destroys plants and Water. The trick of the Defilier, is knowing how to harness this release of arcane energy. The harnassing is a secret the Dragon Kings carefully guard. But when players play Defilers, they should for flavor be vividly destroying plants in front of them, and parching the earth, and sometimes desiccating the living people around one. The players should feel responsible for the extinction of the planet.
For Dark Sun, most of the 5e Druid subclasses are moreorless canon. Dark Sun turns out to handle animism well. The Dark Sun Druids form remote local communities that include the local features of nature, which are inherently psionic to various degrees, such as psionic rock formations, and pools of water that are especially sacred. Any Druid circle can be part of this natural realm.
The Dream Druid that focuses on the Feywild, isnt part of the original canon. But it too can be appropriate. The canonical Positive Material Plane is an area of enlivening Positive Energy that is unlocked where the Elements balance. This is the same thing as a Fey Crossing, and in 4e there patches of the Feywild here and there at these Crossings. There isnt an entire Fey plane or an entire Elemental plane, merely patches here and there like demiplanes, depending on the local circumstances. In this sense, the Epic tier Avangion is Fey, a luminous watery being of harmonious elements in a mode of healing to restore planet incrementally.