And a policy mechanism that lets you define how important a directory (or file) is and how hard the system will work (eg removing less-important data to free up space) to save this.

This idea falls over if the amount of data you create (which the shell can't know in advance) is too big to contain in a snapshot. All that would be possible is an automatic notification that the operations in XYZ directory are no longer undoable due to its size.