Perhaps it would be too great a programming effort, but ideally for me the Library pane would have the following folder capabilities:
1. Primary Folders
3. Selectable Visibility
By selectable visibility I mean the ability for the user to easily show or hide any folder or resource. One of my hobbies is computer-based orchestral music. One of my primary tools is a Digital Audio Workstation (DAW) program called Cubase. DAWs are built around individual instrument tracks, which for orchestral work can run into the hundreds. They share a common problem with Accordance: users find it inherently difficult to navigate a long, potentially complex list of tracks (resources). Cubase has track folders and subfolders to help the user organize his tracks. Accordance users also find this a big help.
But Cubase several versions ago introduced a Visibility Pane which can easily be called up, closed, or kept open by the user. This pane contains a list in order of the folders, subfolders and tracks contained in the current setup. Next to each item in the list is a checkbox. Cubase displays all items by default, but if you "uncheck" an item, it becomes hidden. It's still "there," but you can't see it. If you uncheck a subfolder, all items in it are unchecked. If you uncheck a primary folder, all subfolders and items it contains are hidden. The user has free access to check or uncheck whatever he likes in the entire structure. This makes the program immensely more usable for complex work. You can make visible only what you want to work on at the time and hide what you don't need to see.
Accordance would benefit greatly from such a capability. The similarity of the problem with Cubase is, to me, striking.
Here's a YouTube video tutorial on what was at that time a new feature in Cubase. It will give you a clear idea of what I'm talking about.