Why not just an app that registered a service that was basically a cloud provider for your SD card? Any apps wanting to access the shared storage would use the

Why not just an app that registered a service that was basically a cloud provider for your SD card? Any apps wanting to access the shared storage would use the service's API to request access to a content. The shared storage would really be the private storage for the storage app. Seems like this would be the best of both worlds, no crazy free-for-all external storage but shared data with some type of prompting to grant an app access to a particular path of shared storage.