How to come up with research questions that are well-defined and suitable in scope and relatedness for a thesis? I have decided on the area I want to contribute to, and have found a basic but huge problem for my research. Now my professor told me to redefine the area, so to break the, at the moment, huge area down into subquestions to create an interesting closed problem, which is addressed by my thesis.

At the moment I have come up with several different questions I have found. However, I do not see any pattern where these questions are related to each other. My questions also seem quite loose and do not define the closed problem I am aiming for.

A related question on this site is how to come up with research ideas. My question is not about how to come up with ideas in the first place, but how to define these ideas so that they can practically become a thesis.

Hence, any suggestions how to come up with several subproblems, which define a closed corpus for my thesis?
This sort of thing only evolves as the product of your exhaustive reading, and discussions with your supervisors and peers.

After that, at some point, your supervisor will say something like: "before you can really get into your research, just resolve this little thing - it should be fairly trivial."

That's your thesis, right there.