In my view, the most important thing here is to understand the purpose of a structure:
Structuring a case does NOT mean to tell the interviewer what you're going to look at! This is not a structure - it's just a bucket list... and a pretty random one most of the time!
Structuring a case means to explain to the interviewer the LOGIC according to which you will answer the question at hand. "Areas to look at" are just a byproduct of this logic.
Therefore it is not very helpful to start with defining qualitative "buckets" to look at (e.g., "I'm going to start with looking at the market, then I want to understand the customers etc."). This is, at the end of the day, quite random, with no clear logic from which your areas are derived (other than "experience" or "gut feeling", both of which are not what MBB interviewers are supposed to assess!). If you think about it, this approach is the opposite of how you should work as a strategy consultant. Defining buckets and then hoping to find something interesting in there is pure explorative working - another word for this is "guessing".
One fundamental things that needs to be learned in order to rigorously disaggregate the value drivers of a business and, hence, business-related questions, is how to set up rigorous driver trees. The driver tree allows you to identify the numerical drivers and sub-drivers of your focus metric (e.g., profits). The qualitative elements (such as consumer demand, market structure, company operations, etc.) then have to be mapped to the sub-branches of the tree!
Hence, your analysis has two steps. Imagine you want to run a diagnostic on why profits have fallen:
1. First you need to identify the numerical driver of the problem (e.g., customer base is shrinking). This gives you an understanding of the WHAT has (mathematically) caused the problem (e.g., customer base has shrinked, which you would discover by drilling three or 4 levels down in your profit tree).
2. The second step is the understanding of the WHY (why has this cause emerged?). To do this, you have to examine the qualitative elements that link to the "number of customers"-sub-branch in your driver tree (e.g., competitive situation, market entries, new substitutes, relative price point, customer preferences, product/service properties vs. competition, etc.)
You can think of these qualitative elements as the typical business situation framework elements (see V. Cheng et al.) - but here, they are not hanging in the air, but they are embedded in a rigorous thinking frame which emerges from the disaggregation of value drivers and linking it to qualitative reasons.
Another aspect that is very important (and usually violated in Case Coaching books) is the principle of first isolating the numerical problem driver, before asking qualitative question. Never start your analysis with asking qualitative questions ("First I would like to get a general understanding of the market development" and such phrases)! This is practically the very definition of "boiling the ocean", i.e., working in an extremely inefficient way. First, you should seek to narrow down the area that you need to qualitatively understand - and this can be done very quickly by doing a numerical analysis as described above. Once you know where the problem comes from, THEN you can start to understand the qualitative reasons that underlie the negative development of this driver, and this analysis will be far more focused and concrete than if you would have tried to do it at the start.