Building plugin-based web apps is the way to go. The HN crowd is so vehemently against any added features, that I always wished HN was a plugin based web app itself. Userscripts and extensions are less neat and powerful.
I like audiogata. It is good for creating playlists from different sources. For me, what Spotify does not have, could be supplemented from other sources. This is also the way to go: The true universal music player.
Two suggestions:
- A lot faster way than adding tracks one by one by search, will be to accept a text format (or clipboard paste) of a list of URLs. IMO this is easy to implement.
- A lot tricker, is to allow for simple text file in the format [Artist – Track] to be imported by resolving each line through a search. This is what the, now defunct, Tomahawk player [1] did.