Clearly, many great songs are missing from Spotify and I think your listeners can be of help. After having sent requests both to you and the relevant record companies, I have concluded that this will never work (no action taken, no reply received).
Many of your passionate users have libraries of rare music at home so would it not be in everybody's interest to benefit from this. If users are asked to record these songs at home according to certain standards set by you, they could forward them to you to be published on Spotify. For the work done, I small amount could be requested from the relevant record company as it gives them an opportunity to earn money on these songs.
I know that a very large part of your subscribers are screaming for this but so far nothing have advanced for years.
Give this some effort as everyone can benefit from this instead of sending the problem back and forth to the record companies which are not moving either.