Hey hey,
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' :)
Cheers