This might not be what you had in mind for an answer, but honestly your best shot at making this a good piece of work is by making your dissertation revolve around a topic you enjoy. I did my dissertation on comparing the themes of a number of Byzantine treaties. This was simply as my lecturer specialized in Medieval peacemaking and diplomacy, but had yet to explore Byzantium significantly. Is there any work of one of your lecturers that interests you in particular? Can you think of a new angle, a new period or new theme that has yet to be explored in regards to this lecturers topic? Even if you can't, there is always room for new work on a topic. If you take a suggestion from a stranger on the internet you might find you're not that interested in the topic. This will make you dissertation even more of a drag to write and will also make you 'burn out' quickly. I hope this helps. Best of luck!