> Allow building against packages without installing them.

A partial staged update system would work the best. I should be able to schedule the build of several new packages, have them built into a sandbox, and have new compilations use the sandbox first and then fallback to the system through a union, and then once everything is build finally package up all the pieces and then move them out of the sandbox and into the system at large.

You could make all gentoo updates transactional that way which would be a huge boon in many other ways.