Take inventory of your personal interests within your field of study. Choose a topic or question that interests you. It's hard to write a long and engaging ten-page paper, let alone an 80-page dissertation, when you are not interested in the subject matter.
Identify a research gap and strive to contribute to the body of research around your subject matter. If you do choose a topic that has been written about for the last 20 years, identify a gap in research or results that you have noticed throughout your studies. If there are still questions that linger in your mind about what the previous research has indicated, then it's likely that your questions have not been clarified or fleshed out.
Write down your preconceived notions about your potential question. Whether your notions are correct or not is irrelevant; this exercise is only to demonstrate your inherent biases going into this question and to help you figure out your angle. Even if your research later proves that your notions were incorrect, you have still confirmed or disproved a theory. It is exciting to unearth a new truth, but it is equally important to show additional proof for a theory.
Consider your scope, research tools, and available time schedule. Too wide of a scope is limiting to writers in any discipline; narrow down your question's scope to make it a feasible research project given your time line. Research equipment and materials are also an important consideration when it comes to evaluating your dissertation question. Do you have the materials, sources, access to subjects, and skill set to perform significant analysis on your findings? If not, revise your question or seek further help from your advisor(s) in order to complete your desired project.