First and foremost, it is important to find aspects of your chosen profession that you are passionate about, or at least interested in. Think about the reasons that originally drew you to pursue an education in your field. This may be something very specific, but even if the reason is somewhat vague, it is a start. Once you hone in on what drew you to an education, focus on topics revolving around these interests.
Your professors and mentors may have valuable insight into which direction you should take. More than likely, they will give you some insight based on class discussion, or help you define what topics interest you. They may have an idea of their own, which they are involved in or know is plentiful in research material, or there may be a hot topic in your field that they suggest that you get in on.
Your school or the chair of your committee should have a list of topics that have had considerable research done, making it a relatively straightforward process to complete your dissertation. The topics may not interest you, but you will not have any lack of resources for research and reference.
Choosing a topic that has little to no research done on it may be an excellent idea, if you are willing to put a lot of work into it. Meticulous research must be done on any dissertation, and if it is in an area previously unexplored, you will not have any professional material to reference. This means that you will have to conduct the research yourself. The benefit of choosing an unexplored topic is that by the end of your dissertation you will be somewhat of an expert in the field, and peers will possibly be referencing your work in the future. Your interest in your chosen field will also be a benefit, because you may be more inclined and interested in conducting the research.
A considerable amount of research and work went into crafting your master's thesis, so you already know the subject matter and where to find further information on it. Expanding on this idea may be a good way to get your dissertation started. While writing a dissertation is much more involved of a process than writing a thesis, you will not have to worry about any ethics violations if you use the same idea for your dissertation, as long as you are not copying and pasting any material directly from your thesis.