A crossover with Dark Sun? It is like time travels in D&D, not totally impossible, but very rare. I would be easier to reboot Kaladnay, the dark domain in the demiplane of the dread. Other option would be an almost reboot of Jakandor, where both factions face "visitors" from Athas. If characters from Athas could travel to other worlds, you can bet they can't, or don't want, to come back. If the travel from the elemental planes are possible from Athas, could genies' domains to be "visited" by Athasian refugees? Or maybe the sorcerer-kings have tried to create an army to stop a planar invasion.