For finding a suitable dissertation proposal example, your search can go by two approaches.
Begin with the broadest, most abstract topics in your field, then filter down the most intriguing ones until you have a tangible, testable topic on which you believe you can write three to four papers in two years.
Think of a reasonable next step for a concept relating to your advisor's work or a technique you're familiar with. This could entail using your approach in a new context, innovating how you conduct the technique, or continuing your advisor's work in another way.
The secret to dissertation success is paradoxically to have immense humility about the amount and sort of work you'll be able to complete in a reasonable length of time, but with an enormously bold vision about how that work fits into the big picture and the job you'd finally like to perform.