So I'm not biased or anything. But VMS (real VMS) with the full POSIX support, X Window Systen, running on commodity hardware with source, well, that would be pretty cool.
If I have to make a Franken-OS, I'd like to take the concept of logical names from VMS and bake that in.
For those of you that don't know what that is, you DEFINE a name, to another name, or list of names. You can use that new name like any other name in the system, including the name of a file system. This seems useless on the surface, but the real power is that you can define a name to a list. This could be compared to your unix shell PATH on steroids that have been taking steroids. It is one of the key concepts that lets VAXcluster work. It makes a file system location work like a search path.
I'm a little rusty (is has been almost 25 years) but as I recall, on a cluster node, the logical name SYS$SYSROOT mapped to a list of SYS$SPECIFIC, SYS$COMMON, which allowed the system to have local configuration files that overlayed the cluster's common files. You could set up a logical name for anything that could be named, devices, queues, users, whatever. Very powerful.