Arcane classes have been a task. I want to preserve choice but also preserve flavor. If we have a bunch of arcane spellcasting classes, then it lessens the essence of this setting: magic f***ed the planet up and the only good wizard is a dead wizard.
Yet, if I remove the Bard, Sorcerer, and Warlock, we are left with only the wizard and we've removed all the Charisma-based classes and removed a lot of fun options for players. Later editions rationalized them all away, but each rationalization takes something out of the setting.
So, I'm thinking about keeping most.
Bards become a wizard mutation that found the ability to cast spells when intermixed with music, needing no books. This is not to be confused with the Athasian "Bard," part of a specialized culture in which it is not a surprise, even expected, that even the most poorly dressed minstrel might be a covert agent. The wizards have intermixed into this society, using the role of minstrel favorable to hide their magic. The sorcerer kings are aware of this mutation and tolerate it only when in their control and employ. Any other bards are hunted and outlaws.
Sorcerers I can't justify with my defiling rules, which I'm loathe to dumb down as a centerpiece of Athas. Further, the origins don't fit well. There's only one dragon, wild magic really doesn't fit, divine soul (xanathar) is angels, storm sorcery (xanathar) is more suited for a rainy world. Shadow magic is probably the closest to fitting, but only for those from the Black, making it a good enemy NPC.
Warlocks I can work with. They make great reskinned templars (the sorcerer king can grant powers as it needs), and for players the Great Old One is a dead ringer for Rajaat, the Hexblade (xanathar) sourced from the Gray. The rest, they don't seem to fit the setting (e.g. there are no fey in Athas).