According to Wikipedia, William Durant co-founded the Durant-Dort Carriage Co. with Josiah Dallas Dort in 1886. They leased a factory to build carriages from the Flint Woolen Mills, and the office building across the street is where Durant dreamed up the idea for General Motors - coincidentally, both David Dunbar Buick and Charles Nash, who would open their own respective car companies, worked for the carriage company.
GM will also take over the upkeep of the carriage company's office building. It has been suggested that the boardroom there could become a museum, but publicly all we know is that Reuss says, "We're going to make that area a really special area." Scroll down to watch a video offering a bit of history on the venture, as well as an announcement message from GM's Reuss.