how about the potential danger of car dependent suburbs? what happens if oil prices skyrocket to 10 bucks a gallon in the near future (which i can see happening)?
or here's one...conversation of the environment. just in the last three years here, i've seen more trees and forests come down locally for housing developments which are inefficient. this does not have to touch global warming at all.
or here are some simpler ones: euthanasia, genetic engineering, big vs. small federal government.
i'll come up with more if you are still out of ideas.
You could analyze the different traffic levels in different cities and see what the differences are. You'll probably touch on city size (both population and distance wise), public transportation system, and the system of roads. Not many cities run on grids, with even fewer (if any) doing the most efficient way of having traffic run through (having all streets be of equal importance and size rather than the artery system that is popular and dumb).
It's a question most people don't bother to think about, just complain about.