I want anything that can be used as DLC for Baldur's Gate III. :P

Since all the settings technically exist in the same reality, that could be any of them. But Spelljammer and Planescape would be the most likely settings for an expansion, if an expansion does go to a different setting (also Ravenloft if my theory about the Dark Urge is right)
You Left Out The First Setting That D&D was Created in GREYHAWK That woul be my Pick!!! Lets go Back to the Roots of D&D....
Yeah though to do that setting right, it would just be a dungeon crawler with bizarre pop culture references.