I really like that some apps have options to do that within the app. Apple definitely should make it system-wide though. My Documents and whatever size is always going up.All I ask is that they eventually allow you to delete cached storage stuff from third party apps without having to delete the whole thing. I have a 64 GB and am constantly battling my storage capacity.
It would also be fantastic if you could delete all media from text messages. Having to do it manually per conversation is so stupid.