Inspired by the map here:
http://www.nytimes.com/interactive/2009/03/03/us/20090303_LEONHARDT.html
It would seem that with all the counties already being in your state maps, you might be able to do a version of USA broken into (states and) counties?
It would be nice to have USA/state maps available with other important census geography boundaries such as metropolitan areas, census tracts, census blocks, and zip codes, but these boundaries are subject to fairly frequent changes and realignments, unlike counties. Of course though, any extra level of detail would get used by your loyal FusionMaps customers!