It might be nice to have objects placed on top of existing surface instead of height 0. For example when placing objects on uneven/elevated terrain the object should follow the terrain instead of the ground plane. Maybe it should also automatically go on top of other objects at the same location (e.g. when there's a table the object would automatically be placed on top of the table). You could allow the user to switch between this mode and the existing mode where models are placed on the ground plane (at height 0).
What I'm talking about is actually known as "Follow Terrain" and "Follow Terrain and Objects" modes in some game engine editors (e.g. Cry Engine 3 Editor: http://freesdk.crydev.net/display/SDKDOC2/Placing+and+Modifying+Objects).