During my drive home this evening, it occurred to me that my Astralis, Xana, and Classis Umbra settings could all inhabit the same D&D multiverse if you wanted to run them that way.
You could run the game with all of them existing as parallel empires who connect their material worlds via transitive plane-walking. You could even have them all aware of each other. Perhaps they are at war. Perhaps they all coexist in relative or strained peace.
Alternately, and personally I favor this approach, you could run one of them for a long time, as the sole setting of your game. And then, after a long time, you could introduce one of the others as a new game revelation, previously unknown. How would such an empire react to the discovery of a rival whose methods are parallel?