Combining the Smart List with Custom Sort is a wonderful way to make context appropriate lists for easy and quick task selection. However, not being able to define a custom sort based on specific tags is quite limiting. E.g. distinguishing between tasks that require low mental effort (can be done when tired) and ones which require "un espresso doppio" can be done with tags but then this detail is lost in the list where the custom sort already uses priority, start date, etc.
In summary, adding the ability to group (or even sort) a list with custom sort on a specific sub set of ones tags will be very helpful.