If we're spinning dreams here, someplace in the Hudson Valley would be a good northern terminal for a longer Auto Train route. From a Boston/Mass/northern NE perspective someplace around Albany would be great--the Mass Pike is a much nicer drive than I-95 even before you get to NYC itself, and maybe from the Albany area the train could get down past DC on inland tracks that already have the clearance, don't have catenary, and might be fairly welcoming for one daily pair of trains that don't need to be very fast and that don't have any scheduled stops so they are easier to fit into the flow of freights.
Even people from CT, RI, and greater NYC might be willing, even eager, to start their drive to FL by driving a little ways north, since it would get them around the worst traffic region in the country.
But I kind of doubt that it would pay to buy/lease a site and set up all loading/unloading facilities, unless there's a disused auto-loading facility in about the right place that could be put into service with minor tweaks.