An interesting, if not controversial, subject would be the impact of
peak oil on urban planning. In case you're not familiar with the topic, peak oil is the point in time where the maximum rate of petoleum production occurs, after which it will go into terminal decline.
If/when global oil production starts to fall, what will happen to our cities? Several theories include oil becoming so expensive that no one could afford to drive cars anymore, and suddenly mass transit, cycling and walking will take on new importance. Perhaps the suburbs will become the new ghettos as people try living closer to where they work? What will the impact of more people living in higher density housing be?
I've also read authors who talk about the production and transportation of food. Not being able to rely on petroleum-fueled farm machinery, petrochemical fertilizers and pesticides might mean that food production will have to be done on smaller plots of land, closer to the markets they're sold in. Will we stop importing produce from foreign countries? Will suburban lots transform into urban farms?
There's a wealth of topics you could write about...