Remember, compiling a dissertation topic isn't always about finding new ground. Many successful research careers are built upon challenging existing principles, notions, theories and conclusions.
The best way I have found in selecting dissertation and research topics, is to read further into a particular area you thoroughly enjoy. Once you've finished a book, perhaps select a particular theme, event, or character that interested you most and look at the bibliography to see where the author found their sources. The next step would be to look at those sources and see the controversies, current theories, or even research-holes that exist. Bibliographies create excellent trails for you to look deep into a particular topic to see how it has been developed. This can often built a picture up for you on how worthwhile a topic is to tackle yourself, ie by noticing that most research has focused on this or that fact/perspective, without taking into account or connecting it with XYZ.
In history, there is a debate whether we should let the sources provide us with the questions, or whether we should apply the questions to the sources. Emperical exactness may demand the former, but practical necessity dictates that we should approach a topic with certain questions or hypothesis in mind.
So take a more laid-back approach at first by doing a bit of reading and seeing where your enjoyment and interests might lie.