Problem: on a category page, en:WP:POPUPS already shows the members of immediate sub-cats. However, if these are "tipped open" to show grandchild or lower-level sub-cats by clicking the triangle icon, Popups does not show the members of those lower sub-cats when hovering a mouse pointer over them.
Proposed solution: Extend the functionality of Popups to show the contents of grandchild & lower subcats on category pages.
Who would benefit: Users exploring categories.
More comments: This would be of frequent benefit to participants & admins at en:WP:CFD, but also helpful to all readers who have enabled the Popups gadget.