Looking at the scrapping of tulips, I had always assumed that the issue of an improvement adding a resource to the map, while there could potentially be a later resource (oil, aluminium, uranium) hidden in the same square had proved insurmountable. I would guess that it will be some sort of UB that provides a new resource, so something manufactured that one particular country or group has a monopoly on?
Other than Holly(/Bolly)wood, the only thing I could really think of that isn't tied to an existing civ would be something like cigars... were any native American or African tribes particularly reknowned for producing something unique? Even maybe stained glass?