Spotify should cache any played music locally, no matter if on mobile or wifi.
For this I would like to set a limit (e.g. 5 GB on my SD card) and when full it should replace the oldest cached music (first in, first out).
This way when I'm on a mobile network and decide to stream something, at least it will go into the cache and listening again won't cause more traffic. Or when offline I can keep listening to a new album I just streamed (on wifi or mobile) without having pressed save.
Also pressing save on any music in cache should make it permanent, move it from the cache to the permanent store and not cause additional traffic.
Some of this might already be in spotify's clients, but not as comfortable, transparent or comfortable as I'd like it for iOS and Android.
Updated on 2020-05-11
Your idea was submitted a while ago and unfortunately didn't receive the amount of kudos needed to stay active in the ideas board (100 Kudos per year).
In order to keep the Ideas Exchange clearer I'll close this idea for now, but this doesn't mean Spotify rejected this idea.
If you think this is an idea which needs to be implemented, you're welcome to post it in a slightly different form 🙂
Have a nice day!