Creator of Streamus here. :) Nice app! Feel free to look through Streamus' codebase for ideas if you'd like, https://github.com/MeoMix/StreamusChromeExtension Just give attribution where appropriate.
I like your idea of using Billboard and iTunes charts. While not integrated into the actual UI, Streamus does integrate with Beatport's charts and I have BBC1's charts on my radar. I suppose others could be added as well, but not something I'm looking to do in the short term.
There's actually some semblance of user accounts with Streamus -- they're just not required. I do generate unique IDs for every installation and that can sync across PCs using chrome.storage.sync, but there isn't a lot of protection against malicious activity on that at this point. That is pretty much my #1 priority after I finish ironing out the last of some client-side bugs. I will be adding support for various openID logins and making users who aren't signed-in "second class citizens" while disabling the ability to delete records -- just providing them with the ability to mark them deleted so they don't show up client-side. That way someone can't go on a rampage nuking unauthenticated user data.
Anyway, keep up the good work. Happy to help you with any snares you encounter in building your extension. There's more than a couple 'gotchas' :)
I have been following you and congrats on making the front page of Reddit! You're too awesome. Streamus helped me a lot when I ran into a few problems dealing issues with YouTube player API. Thank you so much for offering to help me like this. I am grateful to have Instant Music noticed by you and I will make sure to keep up the good work and build something that people really need.
Community Playlists - In our office we have a JamBox that one person broadcasts to. We all want to take turns picking the next song, would love to have a turntable.fm style option to invite other users to join a group and collectively manage the playlist.
Lists based on songs on blogs - Would love to just follow some of my favorite blogs recommendations like the ability to load the playlist from Pretty Much Amazing. You could do this by looking for all links that point to SoundCloud, YouTube, etc.
-Add album art
-Ability to search for a song
Greta job so far!