It would be useful to be able to create collections based on the common membership of two other collections, directly from the "Add Rule" button (see attached). This would effectively be an inner join of two or more collections. This option could simply be called "Join Collections".
Of course, this can be achieved using queries within the collection, however doing this at the collection level (instead of within a query) would allow us to create more complex collections much more quickly.
Overall this is a quick efficiency improvement.