I know people have mentioned and voted favorably for an increase in the maximum number of songs that can be saved to Your Music. I know Spotify's acknowledged it, but said it's not in the "current timeline."
What I'd propose, and what might be more manageable, is having the option to purchase an expanded library. Maybe you pay $1 more a month, or a one-time fee of $5 or something. Then you can save a bunch more songs.
I would really, really love something like this. I've maxed out my library, which is saddening for someone who's always discovering new music and listening to old stuff. Spotify says my type of user is in the minority, but I would love for this to simply be an option. In search of more capacity, I've been trying out Google Play and Youtube Premium, but they are in no way as good as Spotify. Plus, besides the song limit, I love everything else about the service (it feels a lot better than supporting Apple Music, which has a limit of ~50,000, I believe).