I am looking for a one line command that I could use to list all deb packages that are missing from a newly installed system, packages that are installed on a reference system (old one).
First, generate the referencelist.txt with
dpkg -l > referencelist.txt on the reference machine.
Then your one-liner looks like:
bash -c 'diff <(dpkg -l|sort) <(sort referencelist.txt)'