I'm missing something - you are just describing what it does already? Replace the word 'folder' with 'app' and that's how it functions now. I know it doesn't seem like it now in the beta but wait for apps to be updated - I've tested this myself whilst playing around with the new APIs.
What is it about a folder that you think will make things any better? If an app is correctly set up - which they won't be until iOS 11 is released and Apple allows them in the app store - then it's documents can be accessed by any other app to read/write files. If you want a 'centralized folder' then just designate that to a particular app (there will be hundreds of these to choose from after iOS 11 launches) and it will function just how you are describing.
The only thing missing is being able to connect the device to a pc and instantly get access to the device as storage - that will probably never change (especially when they are pushing for the ipad to replace such a thing) - but its entirely possible through iTunes or other software e.g. iMazing / iFunbox.
Folders tied to apps seems like a stupid way to go. Why not let the user control where they want to cram their files or how to organize them?