Spotify's vision is to make music discovery social and to connect people through music.
The iOS app fails at enabling users to discover new music in a social way. It would be an easy win to leverage the social network that Spotify has already built to enable collaboration and more social engagement.
Increase session length.
There is a large use case for groups of people wanting to collaborate on a single playlist. This feature would allow people to like, comment, and collaborate on a playlist that could be for a study session, a house party, or to play at work the next day.