Really for a persuasive speech class it really depends on what the professor wants or expects to get a good grade. To do this you need a subject that will be entertaining to your audience, but not push the limit so much that you turn off the professor and the class.
You can push the limit while still being thought provoking. One subject I thought was interesting was whether our Country has outgrown it's current Constitution or form of government. It was persuasive and thought provoking, but since it was an abstract idea instead of something that people already had pre-conceived notions about the student got a really good grade and set himself up for good grades the rest of the semester.
