The biggest restrictions in MorphOS are not the hardware. It is the design of any Amiga-like OS and the necessary legacy compatibility.

A shared memory address space means no way to move to 64 bit memory range to beat that 2GB barrier, even with bank switching. No way to use multiple cores. No way to use virtual memory properly. No way to use "memory protection".

The ideal CPU target, then, would have a single core but a high clock rate. They don't really make those any more.