I don't think the missing piece is a file system
per se. However, there are things a file system does which are currently very difficult and convoluted in iOS.
Take for example the OP's scenario, but let's take it a bit further. Say someone emails me a link to a zip which contains a PowerPoint presentation and a PDF of that presentation, I need to edit this PowerPoint presentation and update the PDFed version, zip it up again, upload it to the server, and email off the link to someone else.
Currently on iOS:
- I get the link in Mail.app, open the link in Safari.app to download the .zip, which triggers Documents.app.
- From there, I can "share" the .pptx to PowerPoint.app, and make edits.
- Except now it's a bit confusing where I have two copies of the .pptx file - the updated one in PowerPoint.app and the original in Documents.app.
- I save the new .pptx with my edits and I "share" it to PDFReader5.app to convert to a PDF.
- More confusing now, I have 3 copies of the .pptx file (original in Documents.app, edited in PowerPoint.app, and copy of edited in PDFReader5.app) and 2 copies of the .pdf (original in Documents.app, new in PDFReader5.app).
- Now I need to be careful to "share" the right .pptx and .pdf to Zip.app to create a new .zip.
- I then need to share the new .zip to Dropbox.app to upload to and generate a link to copy and paste into an email.
- In the end, the .pptx file exists in 5 copies throughout my iOS device (original in Documents.app; edited version in PowerPoint.app, PDFReader5.app, and Zip.app; and in an zip in Dropbox.app).
- This is pretty chaotic, and it's a simple example. Imagine the craziness if I realized I made a typo while zipping it, and went back to fix the typo. I'd have something like 9 copies existing over 3 or 4 apps!
- Just thinking of this makes my head hurt.
A file system solves this by allowing the user to follow the file from app to app. Because each app in iOS is so sandboxed, they cannot access and make edits to the same files and must always make a copy (with the small exception of files saved in iCloud). I'm sure there is another way to solve this problem without re-implementing the nested file system we all think of - but something is needed as today's workflow is a mess.