Hah, I was about to post the same idea, about paying for a course and getting the money back on completion. Maybe even redistribute the funds from prior students who hadn't finished the class to students who complete the class and go back to help others. Get paid for answering questions, kind of like a university TA.

Another idea I had was that students who completed a course, and achieved a certain level of proficiency, would get access to a special forum. This forum could be shared by potential employers.