I too always liked the idea of Wavelist, but Ben and Erin Jo seemed to have stopped iterating on Wavelist some time in 2015. I also like the idea of playlists, but we may be lacking a simple, open format means of creating them however. I seem to recall Kevin Marks pointing me to http://www.xspf.org/ (aka “Spiff”) once as a standard, but I haven’t experimented with it yet.