As it is now possible to both reference Master Measures in expressions and apply Outer Sets to them in Qlik Sense, I would suggest the following idea to better organize Master Measures.
When trying to use Master Measures in practice, you quickly get a long list of Master Measures, which is tedious to scroll through. You might get one Master Measure for Sales, as well several other 'versions' (Sales CY, Sales LY, etc.). When you have the same amount of versions for other Master Measures (Cost, Profit, etc.), this quickly adds up to a long list.
The idea is essentially to create 'derived' Master Measures. You would have one 'base' Master Measure, from which you would derive multiple other Master Measures. Technically, this would only apply an Outer Set to the base measure. However, when paired with the UI feature of an expandable menu in the list of Master Measures, this would make it much simpler to organize Master Measures. See attached example for a mock-up of how this could look.