As a society, it seems like we do not use our space very efficiently. One item is our rooftops. In particular if you drive or take light rail over by Belleview you can look out over the commercial businesses and see all the roof top space that is not serving much of a purpose.
My proposal is to impose a tax for unused rooftop space. If the roof is used "appropriately" as described below, then there would be no roof top tax for that structure.
Granted, this would probably have to apply more so to new development, and to give existing structures a lot of time before the tax would apply.
The goal is to encourage "eco" applications on the roofs. For example, rooftop gardens or parks, solar panels, mini wind generators, etc.
For the rooftop gardens/parks, a city policy change to allow this to count towards green space (open space) requirements could be fairly attractive to developers as they would be able to use more of the site for building development (and offset costs of adding usable things to the roof).