Imagine being able to travel back in time in 3D and really get a sense of what a place was like years ago. If it were ever developed, a great source of historic imagery for modelers would be the Shorpy blog which I have mentioned several times before.
Visit the link HERE to see this image of the Old State House (Boston) in high definition at Shorpy.com. Visit the link HERE to see the Google Earth 3D version of the same building which is still standing today.
In the photograph we see what the area around the building used to look like a hundred years ago in 1890. Very different than now. The neighbors have since been replaced by glass skyscrapers.
A 3D historic buildings layer would include these older buildings, and go back even further to 1790, or 1713 when the Old State House was shiny and new and the neighbors in this photo were decades from even being conceived.
There is currently a 3D Ancient Rome layer on Google Earth that does something like this; but it is a totally separate 3D layer that floats above the modern terrain.