A feature to create and share playlists as well as a way to play music while not on the songs audio page. For example soundcloud's player lets you listen to the music while you browse the site. Also be able to select playlists from this "overlay" (not sure what theyre called)
and the option to download all songs on a playlisgt, so if someone gives you a playlist you can download all the songs in it hasslefree. any songs not available to download are excluded from the download and you are notified of this.
The download comes in a .zip file and includes a .txt file of the licenses each song was released under.
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Option to use simple player. I dont know about anyone elses, but the flash player seems extravagant and sometimes the script "stops responding"
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When you highlight a song browsing audio portal a short, artist defined section of the song is played about 10 seconds long. This will allow artists let users know what the most interesting part of the song sounds like without having to visit the full page.
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"work orders" which audio users can submit and art users can respond to. Things like album covers and whatnot can be "ordered" perhaps a currency such as deviant art's Points can be used for this, or it can just be completely currency free.
collaborations already exist, but perhaps it may be beneficial to tie some kind of reward or point system into it. This is a gaming site. I assume a lot of people like having points of some kind.