I personally would like to see many of the spaces simply be turned back into fields, woods, and natural landscape, rather than trying to discover some profound solution. This is actually happening not so much by design in Detroit where entire neighborhoods are disappearing. Rather than design a new use for the space, many are arguing to leave it and let it be.
Check out this gallery of haunting photographs by Brian Ulrich. The images of vacant malls and big box stores from all over the US are part of a larger project dealing with American consumerism.