Make sure your topic is an argument and not persuasive. With argumentative, you want your side to be right, but you have to prove it by showing why the other side is wrong. With persuasive, you are just trying to persuade others to come to your side.
Think about something you feel very strongly about, but that is not overdone. Abortion, legalizing marijuana, changing the drinking age to 18, puppy mills are examples of things that are overdone. Most of us have strong feelings about these things and could argue and give both sides, but there wouldn't be anything new.