The post pentad status quo really wouldn't be all that bad as long as the deep lore abut Rajaat, the blue age, and so forth is kept obscure/ambiguous in published material. Maybe throw in the vague suggestion of a canonical retcon--but never commit one way or another. Moreover, with a soft reset that jumped the timeline past the book characters' deaths from old age, there'd still be plenty of thematic Darksun to go around.
The take in my headcannon:
Dregoth will have laid to waste and occupied Raam (and most people think he's the original dragon). Sadira will be the faux sorcerer queen of Tyr--less evil but still an eccentric dictator. Tectuctitlay's "son" Atzetuk will be the new divine ruler of Draj, by dint of extreme psychic powers. Balic will be a strife-riven disaster, but with Andropinis still exerting weird extra-dimensional influence. Hamanu, Nibenay, and Lilali-Puy will all be right where we left them. And there's omnipresent religious strife between Dregoth and Lilali-Puy's cultists. All of that is a straight line from 2e's "Beyond the Prism Pentad", with only minimal effort to curtail the metaplot's sunshine and rainbows.
As
@Delazar says, the elephant in the room is easy enough to lampshade. Not every class needs to have a clearly explained and justified canonical origin.
Give all the arcane classes a handwave by stating that Athas is OLD and tons of abstruse sorcerous traditions have accreted over its history. Your PC can be a Bard, Sorcerer, Warlock, or Wizard, but the people of Athas don't have names for those classes; they only know the difference between defilers and preservers. Have a unified set of defiling rules that any arcane PC can use--including arcane tricksters and eldritch knights--and introduce a couple of defiler archetypes that leverage those rules. It never needs to become setting-relevent that the bard PC can use instruments as a spell focus, just handwave him/her as some weird kind of preserver and never have the party meet another one.
Also, allowing an eldritch knight in a Darksun party has the potential to make the setting themes stronger, not weaker. One more PC would have to resist the temptation of defiling and worry about being outed as a magic user--where before those things might only have mattered to the wizard player.
Most other classes are a much easier lift, and a few variant features--like the one
@Delazar used for his paladin player--could be introduced to make them work. Again, with the caveat that a PC can be one but that, like the races excluded from Ravnica and Theros, they have no canonical place in the setting.