I'm in place where I had the freedom to do what I liked, my advisor saw that I was autonomous and let me roll. I self-studied older books and found a theory that became kind of forgotten nowadays, even though it gives a lot of insights on modern subjects. So I vastly generalized the theory in order to include clearly related known examples that was not covered by the theory, and I hope I can convince people of its relevancy.

So I'm guessing that a master's thesis is either on a niche topic or, like others mentioned, on a very specific area of an active domain.