In the user interface where you manage question bank categories it would be worthwhile to be able to collapse/extend the categories.
This should be in the same manner as the edit course/categories UI.
Moodle UI/UX people told me that the edit course/categories is about to get an overhaul. Therefore this idea here needs approval/review from HQ UI/UX.

This was drawn in an initial wireframe on MDL-73209 which isn't accessible any more (has the Balsamiq plugin vanished?) but here is a screen grab.