It can be difficult to decide what to do when you have been given some freedom in this respect. There is also a possibility that the freedom might go to your head so that you take on more than you can cope with in the time available. When deciding on a subject for your dissertation keep in mind the research requirements, - be guided by the saying ‘the narrower and more specific the better’.
Research is a type of learning. When you find out anything, you do it in a particular way, or using a particular methodology, even if you are not aware of it. If you are a third year student, and particularly if you are a Masters level student, you should be aware of the methodology you are adopting in your search for evidence, and of where that methodology fits in the spectrum of possible approaches. For example, it is common to read about quantitative research and qualitative research.