Many players will want it to be totally compatible, for example because there is a dark domain in Ravenloft whose inhabitants are from Athas, Kalidnay.
* Now I am thinking about the idea based in the thread of the intruder and refugees. Beyond Tyr there is a region with new population. Maybe they faked their own disappeance to trick sorcerer-kings and survive the cleasing wars, or from other world surviving a apocalypse worse than the one from Athas. They aren't welcomed and here people haven't ended to understand the concept of coexestence. And the eladrin have discovered "visitors" from a planar gate in the "land within the wind", from the world of Jakandor, or with links to this.
Other idea is the apparition of the vestiges, something like fallen, or wannabe, gods, but with primal magic.