Detroit's biggest problem is that it does not have large enough of a middle class population. That, and half of it is vacant.
Since the 1970s the middle class population has been developing farmland that surrounded Detroit. That process leaves the city core empty while at the same time burdened with legacy costs for infrastructure (which now serves the new suburbs). It is a known phenomon called "donut-holing" or "ringing" and it is enabled by poor planning and easy credit
in the suburbs.
There is one businessman in Detroit that is trying to address the problem. He has a program called Hantz Farms. In a nutshell his plan "was" to reclaim the donut hole and turn it back into productive farmland. He ran into a problem with Detroit City Council due to a law from the 1980s. In general it says that existing farmland within the city limits cannot be destroyed. The issue then, if he converts the donut hole into farmland, is that he cannot easily develop it later. Once it is designated as farmland it is caught in legal quagmire.
The next best use, and the one I think they settled on, is basically park land. The legal construction is something to the effect of experimental use, which allows them to redevelop the land if people move back into the city limit.
http://www.hantzfarmsdetroit.com/