With all the abandoned residential neighborhoods Detroit has a lot of excess land that is unproductive. Some have said turn it into farm land but that itself would require a huge amount of remediation, including demolition and disposal of building materials. There would always be the stigma of vegetables grown in urban soil, plus the growing season is too short.
The solution is to build America's largest solid waste disposal system. With Detroit's excellent transportation system, waste could be brought in by truck and rail from all over the country. The world's largest incinerator could burn it all and use the steam and electricity to power the city. The dilapidated neighborhoods could be systematically dismantled, and the land turned into the world's largest landfill to accept the ash from the incinerator. The existing sewer mains would collect any contaminated liquids from the landfill to be treated at the City's underutilized sewage plant.
The ash should be piled at least 1000' high for the first stage of the landfill, closest to the city, while the second stage landfill is prepared. Then the first stage turned into a ski area, piquing interest as the closest ski area to a major metropolitan city.
Stage two would be 2500' high, and create a world class ski resort, hosting Olympic and World Cup events.
Detroit could be turned into the winter sports capital of the US.