If this has already suggested, then my apologies (but I searched and did not find anything)...
What I would like to see implemented is some sort of enhanced logic that prevents terraforming that causes "gaps" underneath placed/built objects (most notably walls). When a wall is erected and later terraforming is done, to say flatten land, there are times where the ground under such wall becomes "angled" and sometimes even causes a visible gap underneath.
I am simply asking that some sort measure to prevent such terraforming if the removal of dirt, or other supporting material, would cause the land to shift or deform in such a manner. This way we can cleanly terraform up to a placed wall without causing this cosmetic issue.
Right now, the only workaround is to "destroy" the built object, terraform (flatten) the land, and then erect the section again. Considering the amount of materials required for certain walls, this can become quite cumbersome.