If you select a tag (for example, a person or place) in the tag view, you'll get all photos which have this tag assigned to them, sorted by album. But especially if you got a large number of photos and albums it would be quite helpful — and timesaving — to be able to collapse all photos of an album and leave only the album title bar itself.
This function could probably be reused in other places of digiKam as well.
For the gui it doesn't need anything fancy, just a toggle button on the right (or left) of the album title bar would suffice.