Proposal: Qubes OS
Qubes OS is, according to its website "A reasonably secure operating system". Wouldn't that make it on-topic on Super User? Why do we need a dedicated site for it?
Qubes-OS is built on Xen and Linux, but it requires a completely different way of thinking, both in everyday use and systems administration. Due to it's unique structure, most resolutions for Qubes would be completely unusable for just about any other operating system on the market.
For example, because of the way Qubes partitions networks into separate virtual machines, a question may come up about the best way to set up a peer-to-peer network within Qubes. This question would actually contain some security philosophy, as there are many ways to set this particular situation up. What might be a straightforward question were I running Ubuntu, and a question who's answer would probably apply directly to most Debian distros and to many Linux distros... would turn into a very specific question who's resolution ONLY applies to Qubes.
While Super-User might be an acceptable place to ask these questions, even if they only apply to Qubes-land, having a specific Stack related to Qubes would both encourage Qubes users and administrators to more easily find questions to answer and answers to their questions as well as avoid any well intentioned noise about other distributions. Answers related to Ubuntu or Arch or MS-DOS 6 would be completely unusable to a Qubes user.