Design for Resizing Home Directory
When automatic resizing is turned off in homed, the home directories need to be resized manually. This needs design work.
The idea is like this: whenever you run out of space, if another user has space available you get a dialog/notification/whatever stating that "You don't have enough space to do X, but user Y does. Authenticate as them and we'll give some of their space to you.".
There should be a mode where two users can sit down at a computer, open the settings or disk utility app, and re-allocate the space their home directories take on the system.
Creating a new user is another similar situation for storage space.