A setting is not only a group of names of places but it is also iconic characters, heroes or villains. The iconic heroes of DS are the main characters from the saga "the prism pentand", and the iconic villains are the sorcerer-kings, and several don't survive the novels.
You could create new factions or antagonist groups but to introduce iconic villains for DS is different.
And DS needs a special visual look to hook new fans.
Let's remember Athas was affected by the abyssal plague, due the creation of the voidharrow by the god Tharizdum, the chained god.
* Now I am thinking the formians could conquer a lot of domains in other planes, but when this happened those regions were fragmented and expelled, becoming demiplanes or astral domains, and then some formian realms could be within the Grey.
* What if there were dragons in Athas before the cleasing war? The could survive because Rajaat's plan was to be their last target because they were too powerful, but after the betray or rebelion of the champions the dragons disappeared. They knew what was going to happen and they the chose to disapear without any trace. Among other reasons because they noticed they couldn't, or they shouldn't cast arcane magic, or they could be hunted and catched to be used like living-vortices allowing templars to cast divine-magic.
* Could flora from the elemental planes to survive in Athas? Something like the red weed or red creeper from the War of the Worlds. The article "Arcane Botanica" from Dragon magazine #357 says the plants from the elemental planes aren't easy to be cultivated in the material plane.
* Other idea is there is a secret sea within a demiplane. The bad new is its is plagues with biopunk horros like the horror manga "Gyo", something style cyborg-fishes. The good new is these can be eaten.
* What is the status of Kalidmay? Maybe the dark domain is now within the "Endless-Night" (Athasian shadowfell) and this could be a double-edged sword because now there are more space of "ordinary" life but it is full of monsters or supernatural predators. Or after the "shatering" of the demiplane of the dread (I would dare to bet it could be Vecna's fault because he wants revengue against the Dark Powers) now Kalidmay is the "core" of a new demiplane. Who would created a demiplane like this? Divine powers who wanted something like the Wall of the Faithless. It would be a right place for the souls without loyalty to any deity but they weren't too sinner to deserve a worse punishment. Here the divine magic can work thanks the "aspirants" but these can attack each other to steal their divine spark. The penance for the "aspirants" is they have to learn the difference between authority and the true leadership.
