In Civ 6, cities require housing, and one source is from tile improvements (0.5 per improvement): Farms, Fishing Boats, Pastures, Plantations, Camps.
As in Civ 5, citizens can only work tiles within 3 tiles of the city center. However, eventually the national borders will expand further than that. I am wondering if you can add improvements to those tiles to further boost housing.
There would be some precedent for that, since you can obtain luxury resources in this way. I was able to create a plantation to harvest tea in a tile 4 tiles away from my city center, even though I can't work it.