I believe you have a problem in your thinking. It starts with understanding 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.
So one of the worst things you can do is 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 nothing but unspecific and completely random, with no rigorous logic from which your areas are derived. It is, essentially, 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 of the fundamental things that you need to learn 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 or sales). 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. 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. The second step is the understanding of the WHY! 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.