As good as Poweramp's UI is, I think there is one simple thing that would easily make it more useful.
In the folders view (hierarchy or list) each folder entry takes a lot of vertical space (album art if available, name, path, number of elements), additional vertical space is wasted by the "N items; Filter" bar and unnecessarily top title bar. Because of that navigating through the list of folders requires a lot of scrolling (only 5 entries are shown at once on my 5" screen). The songs list use the vertical space much better. I think the UI overhaul in the new version is a good occasion to take care of this aspect as well.