The problem with 'immigration' being the solution is that immigrants move to places where jobs are - which tend to be places with higher home values. While immigration is certainly a net positive for the USA, it doesn't solve regional economic decline. Ed Glaeser has published a series of illuminating articles in the New York Times about agglomeration economies, clustering, and so on that really hammer this point home.
Some interesting recent proposals have centered on providing immigrants green cards with the condition that they maintain residency in, say, Detroit, for a few years. In a strange way, this parallels the
"charter city" concept, in which dysfunctional countries turn over land to other countries in the hope that the new, functional "charter cities" jumpstart systemic reform.
Whether regional economic decline can be fixed by compelling immigrants to reside there is a fascinating question that I'd love to see answered. Does an ineffable "immigrant spirit" transcend regional pathologies, or do depressed regions require much more than that (e.g. massive capital investment) to set off a chain reaction of economic regeneration?