This has been mentioned before, but with Shared Package Containers finally being a thing, it becomes more important...
One thing that we'd like to see is workflows in the packaging tool that involve previously created packages. There are threeimportant scenarios of interest; we use these a lot in App-V Packaging:
Upgrading an existing package by using an installer.
Create a plugin package that has an installer that requires the main package to be installed first.
Creating a package for Shared Package Containers.
In both cases, you want to unpackage an existing package (prior to monitoring mode) to restore it into a system just as it had been originally installed and captured. In the App-V Sequencer this is exposed as "export to local system" as a menu and as part of the "upgrade" and "plugin" scenario workflows.
It is likely that packagemgr has most of the required functionality, but what would make this useful is to embed the functionality right into the Packaging Tool workflows.