A thesis topic should be original and sophisticated knowledge in the field. In advanced university work, finding a suitable topic involves doing a lot of research to find out what has been done and what needs to be done. It should be in an area that fascinates you, not just interests you. After you do preliminary work, consult with your thesis adviser to make sure the idea is approved.
Review all of the courses that you have taken and the books that you have read in the field, and write down the topic areas that gripped you the most. A good thesis topic has to absorb your interest because you are going to be researching, reading and writing about it for a long time. Some people take years to write a thesis. It will be a pleasure to write about a topic that you love; that love will show in the quality of the writing, too. As advisers at Harvard University say, “a thesis topic must spring from your own energies and interests.”
List the arguments that you find experts in your field engaging in. What do they disagree on? These are areas with writing opportunities. You can jump into the conversation and add your research to one side or the other. All professions have their debates, so start your topic research by using search terms in the databases of your field related to words like “conflict," "debate," "argument," "views," "proposals" or "discussion.” Those key words will often appear in the titles of papers engaged at the cutting edge of a field.
Write down thesis topic ideas and then search your profession’s databases for articles on those topics. If someone has already covered your idea, it is not original and you should move to something else. Do not discard an idea too quickly, though. Perhaps those who have written on the topic have ignored what you think are important angles. If you can find a new take on already covered material, you have a topic.
Read the "Conclusions" section of professional papers. Often researchers and scholars will point out areas that need further study, and that is a gold mine of topic ideas. Keep attending lectures, talks and academic conventions. Being part of the discussion generates new ideas. You need to do your own research, but don’t cut yourself off from the academic conversation.
Consult with your thesis adviser once you have a few rough ideas. The adviser should be able to steer you away from stale topics, or even suggest topics. Sometimes his own work will benefit from having writing done in a particular area, and he can suggest that as a topic. A good relationship with a thesis adviser looks like a negotiation, with proposals and counterproposals and reasons on each side.