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 this be clear: it says the packages are installed, but they 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.
Now, how would this be useful? I would like to be able to test the installation of packages on a bare Debian or Ubuntu system. I want to know if a package is installable given a certain scenario. To do this with actual installation would take a lot of disk space and time.
APT has a "simulate" option, but it does not mark packages as installed.