As you are working on array formulas and functions, you may consider the unique aspect of Multiselect data types. I would like the ability to reference a Multiselect data type cell so that the formula or function will treat the values as an array or a range of values.
If the multiselect values are numbers, you could use SUM, MAX, MIN, AVERAGE, etc to perform a calculation.
You could also use TEXTJOIN to concatenate the Multiselect values using any delimiter you wanted. For example, if you had a list of hashtags, you could use TEXTJOIN(" ",ref) to display them in a way that you could copy and paste into Instagram. Currently, copy/paste defaults to pasting a Multiselect as a comma-delimited list.
Array formulas can be very powerful, so interpreting the Multiselect reference as either a single value (if it contains only one value) or an array (if it contains multiple values) could enable power users to do some awesome things.