The Boston Globe summarizes some of the planning to shrink cities such as Detroit and other Rust Belt cities:
The resulting cities may need to look and feel very different — different, perhaps, from the common understanding of what a modern American city is. Rather than trying to lure back residents or entice businesses to build on vacant lots, cities may be better off finding totally new uses for land: large-scale urban farms, or wind turbines or geothermal wells, or letting large patches revert to nature. Instead of merely tolerating the artist communities that often spring up in marginal neighborhoods, cities might actively encourage them to colonize and reshape whole swaths of the urban landscape. Or they might consider selling off portions to private companies to manage.
As the article mentions, some of these plans are beginning to get off the ground. However, I suspect it will be a while before these cities start to look different than they do today. These sorts of plans are usually mentioned for cities that have already “failed,” meaning they can’t find better uses for this vacant or underutilized land. It will take quite a bit of political will (and capital) to admit this and get to a point where residents, business interests, and politicians want to truly pursue contraction.