Here's what I would like:
Start with a virtual system, with no installed packages. Then I invoke a tool, similar to
apt-get to ask it to compute the dependencies and mark all the packages that would be installed as installed.
Let me be clear: It says the packages are installed, but there are no files actually installed.
Then, if I ask for more packages to be "installed", it may propose to add or remove other packages. It wouldn't actually remove packages, but obviously just mark them removed.
This would be useful because, I would be able to test the installation of packages on a bare Debian or Ubuntu system. It would, allow me to know if a package is installable given a certain scenario.
Doing this to an actual installation would take a lot of disk space and time.
apt has a "simulate" option, but it does not mark packages as installed.