It starts with seeing if the law "Every citizen should have a plant." makes sense. The motivation here is to capture carbon dioxide. Then it goes into thinking, is one enough, two, maybe three plants? What should be the size of the plant? What species works best? Can you improve those plants? Then it goes to deciding if wealthier individuals should have more plants than poorer people. What about people who don't have space, can they rent a public space? How would you check if people really have plants? 

This actually is a series of ideas that go along the lines of dilluting the responsability for our planet.