If the U.S. collapsed, a lot of things would happen. An alliance of all the BosWash states would occur and form the new U.S..
NYC would activate its sleeper cells in South Florida and a civil war would occur between South Florida and the Panhandle. Further, more sleeper cells will activate in Miami and portions of Chicago in which Chicago would be incorporated into the BosWash nation and a fence built around the city. From that point, after the liberation of Chicago, all the anti-development Aldermans would be pushed off the Sears Tower, the Sears Tower name restored to its former name, none of this Willis nonsense and a new era of prosperity ushered.
As for South Florida, it will be the Singapore of the Nation of BosWash, establishing connections to other lands.
As for the rest of the former country, eh...