I recently visited Cira Green, a park in Philadelphia that's located on top of a parking garage, about 100' off the ground. The park features numerous garden beds and native trees. There are a number of challenges that come with planting trees on elevated surfaces including soil volume and mixture, ...
Assuming the structure can support the weight, I'm all in favor of green roofs. Though, most buildings are privately owned and therefore are not true replacements for proper public greenspace within a city. I think it's also important not to get distracted from the primary source of land waste within cities: car infrastructure.
Lately I've been thinking about how a city would look if you could take up street and sidewalk, plant trees, and keep just a bike path through the middle. The logistics challenges (maintaining access to buried infrastructure/protecting them from roots, and making sure vehicle-based emergency services like firetrucks and ambulances can get everywhere, handicapped people have options/access everywhere) are keeping me busy but I have a couple ideas for photobashes of solarpunk streets to try out eventually.