Typically, students meet with their committee members and see what they have in terms of topic ideas. Therefore, the ideas you’ll get from other students dissertations are actually products of several scholars’ brainpower—your professors’ professors, and your professors’ knowledge banks. Your professors had to write dissertations in your area too, you know. Also, head to your library and ask your university head librarian to help you find bound dissertations in your major area. Recent ones probably used committee members from your school, and you can see what university scholars at your school are encouraging students to peruse and get ideas.