Retail History
There is an interesting website segment that covers retail history. They write about malls that are vacant, stores that have closed, and some information on growing and booming malls.
I have found two sites that deal with this.
Labelscar is a "retail history blog" that just celebrated its first year of blogging.
Deadmalls.com is all about empty malls and dying malls.
Just another example of the interesting niches you can find online.