This is a little cosmetic, but extra tiles and stuff mostly for decoration, much like the Doodads in games like Starcraft. Would work well with triggers as they could be used to indicate special areas; a farm, a mountain etc. I think they would add a bit of character to scenarios.
Also related, naming of pieces of land, much like in SMAC.
The way to do them is to make them bonus resources. This way you can make a little bit of artwork to customize the tile and you can have it affect the gold, sheild, or food values of the underlying terrain. The editor will know what to do with them and they can even be used in a random map.