First of all The perfect OS will exist only in a perfect world. There will be no competition and no fighting everybody will work together in joy and armony for doing new scientifical and religouse deiscovery. There will be no hunger and now war and no bussines and no work.
The users will also be about 90% programmers and the programmers will also be 90% users.
So no programmer will ever consider helping a "dumb" user because ther will be none. The application will be checked for ergonomy and efiiciency in cooperation by programmers and users everybody searching the mose nice and efficient path to get the job done.
All applications will be written in nice ASM language since there is no need to use bloate code.
The CPU will have a very nice ASM language. Not too many registers but not too little. The mnemonics will be easy and simple to understand as a single glance.
The CPU will not consume much energy and we will not need frequency greater than 500Mhz to run most applications but we could get to 10Ghz if needed by a simple CPU command. Because of paralelization and clear simple design we would never need more than 1Ghz CPU speed anyway.
(You know humman brain / neural network operates and frequencyes as "big" as 100Hertz maximum
more usually at 20Hz but it can fire bilions of operations in the same time)
The CPU will not generate heat and will not depend on pathetic mechanical devices such as air cooling fans to survive. There will be no cooling anyway.
There will be no need for security and protection since we have nothing to fear of. The resources wasted for security and paranoia will be redirected for cooperation and performance.
The cache will be fully under the control of the ASM programmer and it will only help and never get in the way or get things slow.
The memory management method will be your choice and we could experiment new ones not only one (paging) like it is now.
After all there might be only ONE CPU improved by cooperation of all hummans.
The OS will be very efficient, all code will be open and know by everybody. No bloatware or useless stuff. There will be no need for backwards compatibility since any change will propagate too all hardware and software at once or it will not exist.
All hardware and drivers will have clear and simple specifications and interfaces known to everybody.
The OS will be just an OS and the toaster just a toaster. No mixed stuff like a toaster running and OS or the OS toasting the CPU. Each version of the OS and of the hardware will be perfectly adapted for it's task but still compatible with all others.
The whole OS will fit on a single 1.44Mbyte floppy and the "floppy" will have the size of one single humman cell.
Everybody (users and programmers) would have the expertise needed to change the OS CORE code to suit a particular application or usage in very little time.
The best changes will be reviewed by excelent programmers and eventually propagated everywhere if usefull or keept in a global library of reusable code and tweaks for every specific usage or situation.
The GUI could be 2D or 3D or 4D whatever the users wanted with no performance loss. Old things will run as fast as new things. Nobody cared if you would have use 1000 years old software or the latest version, it will all work and interconect just fine.
Every one (programmer or user) will be able to create its own tools and environments to the exact way he/she likes it... but still they will interconect and work together instantly.
AI Neural networks will advise hummans on the best procedures to follow and will run the testings and QA for them pointing out each and any possible flaw or advantage in the design.
There will be only ONE OS anyway
but still specific to each humman.
There will be only Joy, Exploration, Design and Evolution...
No work, no money, no competition, no problems, no murders, no wars, no fears...
Now get back to your Hell ... and try to improve it in the above directions