This stage is important because your interest in your research questions will help carry you through the dissertation process and may become the starting place for future research work or the next step in your career. Be sure to take the time you need for defining and refining your research question(s).
Be creative. This is your chance to experiment with a variety of different questions you might pursue.
Try using various brainstorming techniques (here’s a list of 25 techniques).
Reflect back on your master's thesis, coursework, and seminar papers.
Search digital dissertations and read the abstracts to identify research questions that have already been pursued in your field.
Search a research guide in your field to find relevant articles and focus on the last sections covering recommended further research.
Be careful about research feasibility . . . if you are doing empirical research, your research question must be appropriate for your subjects.
Be careful about broad questions . . . your research question cannot be so broad that you can't adequately cover it.
By the end of this process, reach an agreement with your advisor on an initial set of research questions to guide your research. As your research progresses, you will refine your questions.