I have always been interested in how the cities evolved through the 20th century. Urbanism and urban planning came to the game and changed the way where streets were built, what buildings were built and how public spaces appeared.
Old aerial photos are hard to find, only military or intelligence agencies did some aerial shots before 1950′s. As most of these photos are now in the archives or libraries and have been probably declassified long time ago, Google Maps could browse the archives creating new layers of historical aerial photos on the maps. One could then select a year and see how the cities changed and developed. Animation (slide show) would be also possible, changing the images every few seconds, so you would literally see what and when changed, was demolished and built.