If you could wave a magic wand and suddenly all Unix systems (and variants such as Linux) could be instantly changed to suit your wishes, what would you wish for?

For me, it would be:

    - removal of 'root' user and replaced with various predefined sysadmin groups and standardized audit logs for each group
    - get a batch processing system (similar in spirit to z/OS but without JCL). It should allow you to see a list of the jobs you've submitted, cancel individual jobs, re-order existing jobs that haven't started, have a standard place to see stdout/stderr logs.
    - get something like AIX's smit/smitty for sysadmin tasks
    - have either ZFS built-in or something with equivalent functionality
    - sensible and easy-to-use containerized/jailing capabilities
    - built-in support for append-only file permissions
    - ability to quickly/easily adjust priorities for all processes belonging to a user
    - ability to quickly/easily cap a user's processes cpu/ram resources (without having to use a container/jail)

Solaris from 10 onwards to OpenIndiana/illumos/SmartOS/OmniOS has some of this.

I don't know if you could remove the root user, but you can have a lot of control using pfexec. This might require you to design and assemble it however.

ZFS of course for ZFS filesystem.

Solaris Containers are great: sensible and easy-to-use containerized/jailing capabilities

Solaris has "projects" and the FSS (Fair Share Scheduler) which should allow you to cap in both absolute and relative terms (like a share of CPU time or RAM) on a per-user or per-project/group basis, even if not in a container.

As well, you can create virtualized network interfaces called VNICs and have bandwidth management by VNIC or by port (e.g. port 443, port 25). So you could always reserve say 10% of bandwidth for SSH traffic so you never get starved, etc.

- Accessibility built in, eg the OS takes care of adapting to what the user can do ( vision, subtitles, adaptive keyboard input etc).

- trivially replaceable kernel, to encourage research and experimentation, real time use, etc.

- ruthless, consistent separation of user data, user configuration, and an unbreakable standard for where programs get installed. Just look at dotfiles, hier/LFS, the windows Registry etc to see what a mess this is.

- a native grid compute protocol that allowed any two or more computers to merge into a single system image and automatically distribute workloads across them. We have clustering and the insane complexity of k8s today but it imagine something as easy as "Bonjour and Plan 9"!

At home I had an 8-bit so played around with CP/M and BBS'ing trying to gather shareware (software was too expensive for a kid of course). My first serious experience of "big systems" were VAX 6000 (1GB RAM in 1991!). I'm reminded that many of the useful concepts of that system, some of which you list.. never made it into Unix. One of the reasons I stuck with Solaris for so long later is because it had a few of them.

Sounds like systemd :-).

Interesting. What you are wishing for is, in essence, (Open)VMS. It exists!