Many core processors
I've always wondered why nobody bothered doing this in hardware. You'd have a couple real-time processors set aside for the OS and hardware interfacing, each one of which has its own dedicated memory (Possibly even dedicated chips) and then you'd have all the user stuff run on a huge cluster of standard processor cores. That way the OS is fully protected and immune to even cache-poisoning attacks since it runs on what is essentially dedicated hardware. The Real-time chips could access both sets of memory, but the application cores can only access the shared memory.
A theoretical would system work like the following:
-A user application would just simply send dump a set of requested actions into the shared buffer (EG, I need this file, draw this on in my window, or send this packet to network. The process would then send an interrupt to the OS and the OS looks up the various system calls the process put through by the application process and either performs the action or denies the process based on some security process running simultaneously with the kernel.
-If an application requests a security-sensitive action, the OS itself could halt the application processors and run a check on the requesting process's memory space to verify it hasn't been tampered with and is trusted. If those checks pass, then the request is granted, else the request is denied and the anti-virus engine is called into action. The ability to stop the app cores while the OS still runs would be so very valuable in killing malware or even jsut prevent something from spoofing the OS.
-With enough cores, it could be possible to have every hardware driver run on its own core to interface with its associated piece of hardware. A single real-time core per PCIe lane or other interface would be sufficient to handle a system's needs. Each 'Driver" core would also come with its own bit of memory. This way the system would even be immune to hardware failures, the core running the driver would just need to be kicked. Hell, you could even support hot-swapping the video card...