I have the idea of a fantasy civilization based around shifting dunes. The idea is that the land is largely uninhabitable except for the coasts, but there are large dunes that function as habitable "islands" in this wasteland. Blown by the wind, these dunes slowly drift across the land at the speed of several metres per year, like the somewhat famous Råbjerg Mile in Denmark.
Maybe the soil on this continent is toxic due to pollution by a past civilization so that little can grow there, whereas the dunes carry with them not only sand but also fresh, healthy soil (maybe loess) - and there's plenty of rain, so lots of plants can grow on the dunes.
These dune-dwellers are somewhere between settled folk and nomads, since it can take years for a dune to move a distance equivalent to its own length.
There is a problem, however: Råbjerg Mile only keeps moving because of conservation. You can arrest dune drift by building fences and planting the right plants.
If my dune dwellers can just stabilise the dunes, that's no fun. Then they'll soon be just like any other settled folk. This is only fun if there is a good reason why they cannot lock down the dunes.
Having a superstitious taboo against stopping the dunes is not good enough. It's not satisfying for me to have a culture built on stupidity. That also sounds unstable. I need a better reason than that.
It's important to note that this is fantasy, not science fiction. Supernatural explanations are acceptable.
Thanks in advance!