Summary and Info:
I am suggesting a "recommends" (or recommended, etc) field to suggest packages that would be installed together but can be removed afterwards (or by specifying an option to ignore).

Use cases:
1. To reflect upstream recommendation.

For example, Python suggests that setuptools and pip would be present on a Python installation. It should be present for most users, and the others should be able to remove it.

Same for Node.js and npm.

2. To provide a working package by default, but allow for customizations.

For example, currently nvidia depends on libgl which defaults to mesa-libgl ( FS#43119 ), and it won't work with a typical nvidia installation. This can be solved by making nvidia "recommending" nvidia-libgl.

Also ca-certificates is currently using a hacky way to emulate this behavior, where ca-certificates (an empty package) depends on ca-certificates-mozilla and ca-certificates-cacert, and those two packages depend on ca-certificates-utils, which provides ca-certificates, so the user can uninstall ca-certificates together with ca-certificates-cacert if he doesn't want cacert without breaking his dependency chain. This can be simplified to ca-certificates "recommends" the two packages.

Also some package needs a "one-of-the-two" dependency, this is also a simple way to achieve it.

Same for python-matplotlib ( FS#41744 ) and probably others, too.

Behavior:
It's mostly like optdepends, like a warning on removal and shown on related info outputs. The only difference is that the recommended packages will be installed by default when installing (not upgrading) a package, and there should be an option (command line and config file) to disable this.