You will be living and breathing this subject for a good portion of the semesters, so pick something you care about or something you need to know about. You don't have to choose a 'traditional' legal topic. In fact, when we are grading the Pathfinders we give more weight to topics that are not already covered by the standard resources, or interdisciplinary subjects and are therefore more difficult to research.
Most topics cover more than just cases, statutes, and journal articles. Law practice is rarely confined to traditional legal topics so try to think broadly about your topic. Would statistical information, technical information, etc. be helpful to someone researching in the area?