Take a look at Gothenburg from above. Like most western cities it is pretty grey. Roofs, streets and squares are all covered with inorganic materials.
Just the rooftops constitutes one third of the city.
What if this area could be used better?
On the flat roofs and in parks crops could be grown. On southfacing roofs solar panels could be mounted. Small windmills could produce electricity. More apartments could be built on some roofs.
A network of sustainable highways could connect the city. They would work as personal rapid transit systems and shelter cycling highways. The sustainable highways could even collect energy and water.