I've maintained for a while now that the Downsview Airport site would be the perfect location for an Olympic Park and/or NFL stadium, combined with other redevelopment efforts. Intersection of 2 major highways (if you count the Allen as a highway, which is mostly is), and the potential intersection of 2 subway lines, AND a GO station at the north end of the site. In terms of transit and road connectivity, it doesn't get much better than that. Oh, and it would have a killer view of downtown.
Of course, removing Downsview would also lift a whole host of height restrictions in the surrounding area, allowing development around the Spadina line and future Sheppard West line to densify.