Transactional ACID updates to file systems would be pretty fantastic.

BTRFS has had a bunch of problems trying to actually compete with traditional filesystems, though. In the distributed world, CalvinFS seems pretty promising to me.

I always wanted the file system to be built on a version control.

Also MUMPS. See https://en.wikipedia.org/wiki/Pick_operating_system for info.

In some ways, Microsoft is on the way there with the way PowerShell works, and the ability to script things through OS functions that return objects which can be queried. If we ever see a WinFS, it would be very powerful combined with PowerShell