A few features that I would be happy of (Deadhead plugin, support for Discogs, support
The features I'm interested of are the Deadhead plugin, it was available for Winamp and it's quite nice, plays Grateful dead band's livesets and also a few other bands (Furthur, Mikckey Hart band, Phil Lesh and friends and Ratdog) - https://sites.google.com/site/arodsisland/deadhead-help and https://www.facebook.com/Deadhead-The-Grateful-Dead-Plugin-for-Winamp-160932870610751/
This plugin plays the band livesets from archive.org.
Support for Discogs and support for Wikipedia also to get artist and album information or maybe for a feature to it for work like it used in Clementine.
What if you added support for Spotify, Deezer, Youtube, Vimeo, Soundcloud, Mixcloud? It would be kind of a superplayer then!-)
With Spotify or Deezer support I mean that the player could have a simple search bar for them without unnecessary graphics and other bloat. Youtube and Vimeo support might have an option for the video to watch. Soundcloud and Mixcloud, these also wouldn't need more than a search bar in my opinion. You would need to use your account in all these streaming sites.