The way that Aimp displays files is nice.
It would be nice Kiss Player show the files categorized by their folders.
Also an option to work with tabs.
This seems nice, but as an optional feature (as personally I wouldn't use).
It can be tricky to implement with FLTK, one must verify if it's possibly to do with FL_Tree or a custom widget.
Please create another bug for tabs and specify how it should work.