On a mature server there are plenty of scars in the landscape, I can only assume, made by people training farming (ploughing). I think it is real-life enough to expect the grass to grow back - at least within a game year. (And soil to revert to fertile soil?). I know some people want dug soil of all kinds as a color tile for their build, so could the auto-grow occur outside claims?
This leads me to think that deserted cities could be more attractive if the mounds left are allowed to grass over. It would also be natural if the flat ground and other flattening actions were 'released' so the contours were naturalised, creating a more slumped ruin. A deserted site looks too much like a building site IMO. I'm not suggesting any auto-terraforming - If a new guild wanted to use the site they could simply flatten again.
Laid stone and marble should persist indefinitely as per usual. Permanent roads important.