Coming up with a topic

As a part of my degree, I am supposed to write a dissertation. I had always assumed that writing a 100+ page document would be the hardest part. I have learned that I am way off. For me, the most difficult part is coming up with a relative, interesting topic.

I've had a couple of ideas that I have gone through without much success. Perhaps it has to do with my initial lack of understand of what a dissertation is. A little research brings me to Purdue's definition (somewhat paraphrased):

A dissertation is a lengthy formal document that argues in defense of a hypothesis. The research performed must be original and substantial and that its essence is critical thinking and not experimental data. Every statement in a dissertation must be supported by either a reference to published scientific literature or by original work. Each statement must be correct and defensible in a logical and scientific sense.

Before I started talking with professors, I had the grand idea of writing an application that would data mine publicly available social networking sites and develop a somewhat intricate picture of the person a forensics team was evaluating. However, that is just software development. It's not moving the science of forensics ahead, so the school's advisers shut that topic down.

The I realized that children of today are getting on the internet at progressively earlier ages. However, parents and schools are frequently not equipped to engage children and explain to them the dangers that could lurk on various websites. However, what Purdue's document doesn't mention is that completing a dissertation is frequently a lock into a specific career field. And I don't want to be the guy that teaches Facebook to elementary school for the next 30 years. That topic even came up indirectly with a former professor of mine:

@itsme_brian what are you interested in and willing to live with for life?
— Sam Liles (@selil) February 16, 2013

Now the goal is to identify a topic that I can write about, enjoy, and make money on during the next 30+ years of my working life. Ideas?