T A big part of Athas is the duality between the preserver and defiler wizards and plants being the source of arcane magic. The warlock doesn't really fit due to the pact magic thing, arcane bards don't exist on Athas either, and only the wild sorcerer makes much sense as well. Moon Druids would also be a bit meh due to a good chunk of the critter they can turn into not existing.
They shoehorned stuff into the 4E setting, the conversion was meh on multiple levels. There is room for new classes on Athas IMHO but I odn't think they shoul muck up a good chunk of the lore to fit them in (Paladins, Warlocks, most Sorcerer subclasses/Sorcerer itself, arcane bards).
In 4E terms most classes tied to the arcane power source should not exist on Athas or if they do NPC only IMHO.
Same thing with the races. Gnomes and Half Orcs should be right out, Dragonborn, Drow would be other exclusions.