If you are a PhD student at the point of beginning your dissertation research, you must have some idea what you would like to study. To get ideas of the kinds of topics typically researched by students at your school, go to the library and browse the section where all of the dissertations are housed. You might also ask your adviser for a list of the titles of the dissertations of recent graduates.
Once you have a topic and have conceived how you are going to research that topic, we can help you with the process of writing your prospectus and, eventually, your dissertation. But any ideas we, who are not expert in your field, might have about topics would not be particularly useful to you.
Simone, EssayForum.com