Enter two cities to compare:
1) I would like to transition the app to use the census API found here: http://www.census.gov/developers/
2) Eventually I would like to compare cities by taking the actual land area of one city and expanding/shrinking the city limits to the land area of a different city and measuring the population from those new city limits.
Some hurdles would potentially be: Expanding a city in an intelligent way so as to avoid uninhabitable area, getting the population of an area defined by a polygon and maybe getting the population to enough detail.
After some research I believe this may be theoretically possible by expanding/shrinking the city limits in the direction of population centers of the surrounding area. For example, if you looked at a choropleth map of population density you would easily be able to tell in what direction it logically made sense to expand or shrink a city while avoiding uninhabited areas. It also appears that census data is accurate down to the block, which should certainly be sufficient for these purposes.