Adding new systemd service, for example just requires me to put a file into `/etc/systemd/system`. Many other applications work like this as well: Apache configs, /etc/sudoers.d on Debian, more examples in `/etc/?.d`

It's generally a good practice to make your system open for extension but closed to modifications [1].

There is no reason, the user database can not work the same way. Just have the system scan `/etc/passwd.d/*` on boot/reload and drop files there.

[1] https://drive.google.com/file/d/0BwhCYaYDn8EgODUxZTJhOWEtMTZ...

User management could be handled as part of the package metadata, rather than an install script.

Schema migrations would still need a script or something though.

Then you need to add support for this features to the package manager, I imagine that the package manager will never cover all uses so you would need for rare cases the ability to run a script.

That script could be sandboxed to only have access to its packages' files.