Below you can find a list of the most common case types and some high-level recommendations on approaching them. Have in mind that the case might have a mix of several problems (e.g. the profits are going down, but the problem is in operations and thus a profitability case transforms into operational math problem):
1) Market sizing - structuring from the supply or demand side. Structuring using a formula or using an issue tree
2) Profitability - basic profitability framework. Remember about different revenue streams and product mix
2) Market context cases (Market Entry, New product, Acquisition, Revenue Growth, etc). Always start with the big picture "market". Finish with something specific to the case (e.g. How to enter?"). Structure it as if you are defining the work streams for the real project. Try to get the quantitative data first.
3) M&A / investment cases - Due Diligence cases (basically acquisition cases from the previous one), Synergies - revenue synergies (price, qty, mix) and cost synergies (value chain). Investment decision - assessing the attractiveness of the business and financial side of the investment. Also, remember that Valuation may be a part of any M&A case
4) Operational math problem (e.g. Should we increase the speed of an elevator or just buy a second one? How should we reduce the queues? Etc.) - Structuring as a process / value chain, with inflows, operations, and outflows
5) Cost cutting - I provided the recommendations on structuring it here: https://www.preplounge.com/en/consulting-forum/operations-cases-mck-1105#a2134
6) Social / economics cases - huge variability:
- Factors influencing the price (What are the factors influencing the price of oil? Factors influencing the price of real estate?)
- Macroeconomic cases (How will you improve life expectancy in a particular city? How will you decrease the unemployment?)
- Public sector non-profits (Increasing the revenues of a museum; increasing the revenues from tourism in a city)
- Redesigning the processes (How will you develop a new anti-monopoly regulation?)
- Non-profit investments (A billionaire is building a new school. What are the factors to look at?)
This is just for inspiration. You should ideally make your own system.
Best of luck!