Music play history
It would be really useful to me to have a feature in which every track I play gets recorded (optionally, if some users don't want it) and I can then look it up later. For example, sometimes I'm playing through a really long playlist or album and I take a break to play something else and I lose my place inside it. Once this is built, it could also support other features like "play insights" e.g. what genres you listed to the most, the most listened to song, etc. This "play insights" feature is far less important to me but might be cool.